The 10 essentials are essential to have on every hike.  Why?  Well, they have been proven to be the minimum of what you need in order to have a successful hike, and to survive an unexpected night in the wilderness if something goes wrong.  Here I'm going to go through them and explain what they are and why they are so important.  I'll also talk to some of the seasonal changes that you need to make to be successful in winter, summer, and the shoulder seasons.  The 10 essentials are:

1. Navigation
2. Insulation/Rain Gear
3. Light
4. First Aid Supplies
5. Emergency Kit
6. Fire
7. Nutrition
8. Water
9. Sun and Insect Protection
10. Emergency Shelter

Navigation:

In essence, navigation is how you plan to figure out where you are and where to go.  Traditionally this is map and compass, but with the advances in technology there are now dedicated GPS devices, and a significant number of people, myself included, use an app on their phone.  Navigation though is something that must not fail for you, because when you need it, you need it to work the first time.  This is why you should always carry a paper map and compass and be aware of about where on the trail you currently are.  You should also know how to use a map and compass plus whatever digital navigational system you prefer to use.  I have seen a number of times where someone admitted after the fact that they didn't know how to use their phone's GPS and had to figure it out in the field.

I haven't heard of, or when I used one decades ago experienced, issues with GPS systems failing horribly.  They tend to have reasonable battery life and its possible to find models that still accept batteries.  The device I currently use is an InReach Mini 2 which is only rechargeable batteries and requires a phone to really use the map features, but even with it sending the tracking information back to Garmin in sub-freezing temperatures, I have not had it fail on me yet.  Of course, the more you use the screen on these devices, the faster the battery dies, so on devices you can actually see the map, they may die quicker than the typically advertised "24+" hours of use per charge.

Phones on the other hand have issues.  They're super convenient because we all have one, and downloading a free app is easy enough.The problem is, the battery dies quickly, especially for older phones or in super cold temperatures.  If that happens, and that's your only means of navigation, you have nothing.  Similarly, modern phones are fragile, and breaking them is quite easy, same result, you have nothing.  Water also kills phones, especially if you drop it in a stream crossing.  And lets not forget we could drop it down a crack and not be able to retrieve it.  With the exception of loosing it, a dedicated GPS device is much more durable and is designed to survive these situations.

The other issue with phones are the apps we use.  When we're in the middle of the woods, we often don't have cell signal.  That means we really need to have downloaded the map prior to the hike.  If you didn't, you may not have a map.  If you're using a free version of the software, you may be able to only download your intended route, and all the other trails in the area are hidden from you.  This means that if you're not tracking your hike and you go off the correct trail, you may be lost and not know how to get back to the intended trail.  All this is just a word of warning to know your software and its limitations before you go into the woods.  You may want to upgrade to a paid version, or switch to a different software that has all the features you need.

One final thing on phones.  As I mentioned, they can really eat through batteries.  There are a few things you can do to help save the battery.  The first one is to turn on airplane mode when you start the hike.  This will not turn off the GPS, so you can still use your phone to track your hike.  Instead, one of the biggest battery usage is the phone searching for a signal, specifically cell signal and WiFi signal.  Turning on airplane mode turns off those features and the phone won't be searching needlessly.  Bluetooth can also sometimes be kept on in airplane mode.  This is helpful if you've got a fitness device on you or are using something like the InReach Mini 2 that connects via Bluetooth.  This doesn't drain the battery as much as searching for signal.  The final battery saver thing is to turn down screen brightness.  It's not going to save a lot, but it may get you a couple more minutes.  Obviously, the less you use your phone, the longer the battery will last.

Insulation and Rain Gear:

This is one of the things that can change depending on the season.  We first need something to keep you dry when weather happens.  This is typically waterproof jacket and possibly pants.  In the summer, I know that if I'm wearing a rain jacket, I'm getting just as wet from sweat as when I'm not wearing one, so I will often just get doused in the rain.  The issue with that though is I can cool off too much and start to go hypothermic, so it's a judgement call when to use it.  In the shoulder seasons and winter though, getting wet can be deadly.  So if there's rain or heavy snow, wearing a waterproof jacket is essential to stay alive.

The insulation part comes into play during shoulder season and especially winter.  You will want to have with you items to keep your head, hands, and feet warm.  It's possible, especially on top of mountains in the winter, that you will want goggles and face masks.  You'll also want to start dressing in layers.  The crash course on layers is you want to think of them as 3 parts.  Base layer that wicks moisture away from your body, Mid layer that provides insulation, and Outer that keeps wind and water out.  When hiking you want to keep yourself slightly cool so that you keep your sweating to a minimum.  When you are starting to feel hot, take a layer off, when you're starting to feel cold, add a layer.  Whenever you stop for more than a quick break, add layers, and take them off before you start back up.

Now that we know what we're supposed to be wearing, think of backups.  Always pack an extra set of socks, maybe 2 sets if its going to be wet. There is no better feeling than changing into dry socks after hiking in wet ones for hours.  In the colder months, you will want to start carrying backup layers.  In the deepest of winter, I always carry a full change of clothes with me in plastic bags.  There are 2 main reasons I do this.  One is I may fall through ice and get totally soaked.  It's not unusual to hike across lakes and up frozen streams and rivers in the winter.  If the ice is thick enough, it's safe to do.  There is always the possibility you find a weak spot and fall in.  If that happens, you want to be able to change into dry clothes.  The other is if I have to spend the night.  Sleeping in dry clothing is warmer than wet clothing, so before I would go into the shelter for the night, I would change into dry clothes.

Light:

Dedicated flashlights are a must, especially in the colder months when there is less daylight.  Growing up, it was a handheld flashlight, and they still exist and are still useful.  Today though, we're usually talking head lamps.  The advantage of head lamps is your hands are free, and the light will shine where you're looking.  Just be kind to your fellow hikers, and angle it down so you're not shining it in their face.

We also should be carrying 2 light sources.  These are all going to use batteries, and batteries die.  If you are one that wants to use a rechargeable light, you should have a backup that is not.  You also need to be carrying backup batteries for your lights, so when it dies, you can still have a working light.  Another reason to have 2 is for changing batteries.  Trying to get 3 triple A batteries into your headlamp in the dark is very difficult.  Having light to shine onto it will make it significantly easier.  We also need the right type of batteries.  For summer use, the regular alkaline batteries are just fine.  For the super cold weather, you're going to want lithium batteries as they keep their charge better in the cold.

Notice I did not mention your phone as a flashlight.  Yes, it does have one, and if you need to change batteries, it'll work so you can see what you're doing, but it's not a solution.  Using your phone will only take something that is likely low on power and kill its battery.  There goes your light source, but also your means of communication in the event you need a rescue.  For this and all the reasons I talked about phones under Navigation, you should not rely on your phone.

First Aid Supplies:

This should be an obvious one, if you get hurt in the back country, you need to have something to patch yourself up.  The vast majority of injuries you will face are cuts and abrasions, so you should have bandages to deal with those.  You should also have disinfectants and basic medicine like ibuprofen (Advil), acetaminophen (Tylenol), diphenhydramine (Benedryl), and know when and how to use them.  There are basic kits available through most outdoors stores that will give you a good selection of bandages and OTC drugs for your outing.  These are not complete kits, but they're a great start.

There are many things you may want to add to your kit, and this leads to compromising between space/weight and the need for the items.  Thankfully most are small and lightweight.  For blister care, you will want moleskin or a blister specific pad.  I've also found Leukotape works extremely well for covering up blisters, or to use as tape to hold on bandages, it's much stickier than regular medical tape.  For sprain/strain care, you'll want to add in a couple elastic bandages (Ace bandages).  For breaks you'll want a SAM splint.  You'll also find that triangular bandages.  You could keep adding more and more things, but these few additions will significantly help you in the event you need to use your kit.  

The other thing that goes with this essential is training.  At minimum, I highly recommend taking a first aid and CPR class through the American Red Cross or American Heart Association.  This will give you the basic knowledge of how to respond.  If you are finding yourself out in the woods more and more, you're going to want to up your training.  There are various organizations that offer Wilderness First Aid (WFA) and Wilderness First Responder (WFR).  The first is typically a weekend class, the second is typically a week long class.  These are great in that they teach 2 major concepts.  First is how to improvise the gear that you need.  The second is how to manage whatever the issue is longer term (many hours to a couple days) so you can extract the person.

When having to do first aid, it's tempting to use the injured parties gear.  This saves you money as its not your stuff.  The down side of that when you have a bigger emergency is they most likely will have only a few bandages and maybe so disinfectant.  Yes, if that is appropriate for their injuries, by all means use their gear.  But if you have a good kit, you'll probably have more stuff that will easier to use.  More importantly, you know where your kit is and what is in it, so you're not wasting time trying to find their stuff.

One last word on first aid kits.  Stuff expires.  The drugs, antibiotic creams, they all have expiration dates.  Keep an eye on them, check the kit at least once a year if not more often, and replace expired stuff before it goes.  In most cases, expired stuff will still work, but they may not be as effective.

Emergency Kit:

This is your repair kit and methods to signal for help.  On the signal end, a whistle, signal mirror, and bright colored cloth.  You should have at least one of these.  The good news is that if you use an Osprey pack, the buckle for the chest strap doubles as a whistle.  You probably can improvise a signal mirror with a watch or your phone where you reflect the sunlight to the aircraft that is searching for you.

The repair end of things can often be boiled down to a knife or multi-tool and duct tape.  With the duct tape, the easiest way to carry some is to wrap it around your water bottle, or some other round thing you're carrying.  You can also just wind it up around itself to make something more compact.  Just be aware, duct tape does go bad, so you will want to replace it every now and then.  Most things that break in the back country can be taped back together well enough for you to get out of the woods.  Not always though.  Some other things you may want to consider carrying are small zip ties and paracord.  The zip ties, if small enough, are great for repairing a popped rivet or a broken link in a chain, things you're likely to see in the winter with snowshoes and microspikes.  Paracord will help you improvise larger issues such as when a pack strap fails.  These are bandaids for your issue.  If you're repairing important gear like boots, packs, snowshoes, microspikes, etc. you should be heading out.

Fire:

When we're talking fire, we are talking about the ability to make one in an emergency situation.  The minimum you will need is a lighter or matches.  Be aware though, lighters fail quite easily if there is rain around.  I tried 3 different ones once to light a stove and all failed in the field.  Matches in a waterproof container make a great backup.  With this, fire starters are another thing to bring along.  If you had to, you could raid the first aid kit for paper to burn, but a dedicated fire starter is preferable.  Along with all of this, you need to know how to build and light a fire using stuff you find in the woods.  It's not hard, but it does take practice.  One thing that will help falls under the emergency kit... your knife.  This will allow you to split small twigs into smaller kindling and make lighting a fire easier.

Nutrition:

Food is a very important thing to carry.  You want to have enough to keep yourself running if you get stuck out for a night.  Yes, you can live for a while without food, much longer than without water, but if you are hungry and your blood sugar drops as you're exercising and not replacing what you've used, you will be in trouble.  Similarly in hot weather, if you're not replacing the salt you are sweating out, you will be in trouble.  Both low blood sugar and electrolyte imbalances can be deadly.

The most important thing about food is that it is something you will eat. If you don't eat tuna, don't pack it.  You do want to have something with you that is high protein, high fat, and high starch/sugar.  They don't have to all be in one food, but you want to have all 3 with you.  You will also want to be sure that you're eating throughout the day.  I am bad about this myself, but on the longer hikes, and especially on winter hikes, I will force myself to eat on the trail.  It may be only trail mix (my preference is dried fruits that get the sugar/starch and some protein), but it is still something.

One hike I did during the winter, someone in the group I was hiking with only ate sugar all day.  Literally.  They were a slow hiker, and they stated afterwards that they didn't want to slow us down any more by stopping to eat real foods.  What happened is they bonked on the way out.  They started wandering off the side of the trail, and were very slow on their way out.  We only found out afterwards that they had plenty of proper nutrition in their pack, they just never ate it.  

Myself, I have bonked too.  I know my body well enough and I recognized how I was feeling and that I needed to eat something.  I had eaten my dried fruits through the day, so I knew it was a matter of needing proteins and fat, so my backup can of tuna came out, 4 miles from the car, and took about 2 miles (about an hour) before I felt normal again.  It's scary when that happens.  

It is extremely important to pay attention to your nutrition and make sure you eat the food you need when you need it.  When hiking with a group, don't worry about slowing the group down.  Your personal well being is more important than making the peak.  

Water:

Water is not only what you carry on your back, but your ability to produce potable water when you run out.  What you carry should be more than you think you will need to get to the not only the next water source, but the one past it.  You never know if the water sources are truly reliable, especially if it's a drought.  Every time you pass a water source, ask yourself if you have enough water, and make water if you don't think you do.  Water is heavy, but it is well worth the weight to carry it.

There are many ways to produce potable water.  The way that weighs the least is to us chemical tablets.  These usually are add a couple tablets per liter and let it sit for a certain amount of time.  There often companion tablets that you add when the water is safe to remove the taste of the chemicals.  Filters are usually preferable.  Some like the Sawyer brand you can attach directly to a bladder or bottle and squeeze it to filter it.  Others, like ones made by MSR and Katadyn, are pumps that you drop a tube into the water source and a tube into your bottle and just pump it through.  Finally, and most useful in winter, you can boil the water.

In an emergency, don't let the fear of getting sick to stop you from drinking unpurified water.  Lack of water will kill you now.  Giardia or anything else you will pick up from a pond or stream won't hurt you for days or weeks, and you will hopefully be out of the woods by then and able to start treatment.

Sun and Insect Protection:

I have to admit, I do laugh at this one, but it is important.  Insect protection like DEET and physical barriers like nets are the best way to keep bugs away.  For the most part, bugs will just be an annoyance.  That said though, diseases can be transmitted.  With mosquitoes you have West Nile virus.  With ticks you have Lyme, Anaplasmosis, and many others.  These all are not immediate issues, but they can be long term issues.

Sun protection can be both acute and long term issues.  Long term, you're talking about skin cancer.  Short term, you're talking about burns.  While sun burns are annoying, they're not a huge issue immediately.  If you have to spend an unplanned night in the woods though, you may run into issues with temperature regulation.  It is best if you can prevent the burns with sun block and appropriate clothing and hats.

In the winter, bugs aren't an issue, but the sun still is.  Snow can reflect the UV rays back at you and still cause burns.  The other side of it is that this can also cause snow blindness.  So, even in the winter, sun protection is important.  Don't forget the sun glasses and sun block, even on cloudy days in the winter.

Emergency Shelter:

This is what you will use to survive an unexpected night out in the woods.  The easiest thing to carry is a space blanket.  This can be used to wrap yourself up in, or with the paracord or duct tape from your emergency kit you can use it as a tarp.  There are also now space blankets that are already pre-formed as bivy sacks that fit 1 or 2 people in them.  In the end, you want something that will keep you dry and warm in the event you have to spend the night.