The "10 Essentials" is a list of items. It's been around for a long time, and has evolved into the one presented here. According to the editors of "Mountaineering: The Freedom of the Hills", "the purpose of this list has always been to answer two basic questions: First, can you respond positively to an accident or emergency? Second, can you safely spend a night - or more - out?" Having the items on this list, combined with some basic knowlege and calm thinking, will go a long way towards making sure that you can.
Ten Essential "Systems"
Navigation (map and compass)
Sun protection (sunglasses and sunscreen)
Insulation (extra clothing)
Fire (waterproof matches/lighter/candles)
Repair kit and tools
Nutrition (extra food)
Hydration (extra water)
Let's look at each of these separately. Keep in mind that the contents of this list will vary depending on the situation and time of the year. For example, extra clothing in the winter may mean balaclava, mittens, and down jacket, while in the summer a simple windbreaker may be enough.
Map and Compass - If you're hiking in a local forest park, then the simple map you can get at the visitor center is really all you need. In unfamiliar mountains you may find that a topographic map is necessary. If you know how to use it then a baseplate compass is well worth having (I carry a Silva Explorer Pro), otherwise, a simple keyring compass can be used. The key here is, of course, knowing how to use a map and compass. A GPS is useful, but a map and compass are lighter, cheaper, well proven over centuries of use, and don't rely on batteries to operate.
Sun Protection - self explanatory. Sunglasses for glare and sunscreen to prevent burning. A long-sleeved shirt and long pants are also options. I'll add insect repellent here too. Deet works better than anything else, and if you're concerned about rubbing chemicals on your skin, the ideal application is lightly spread over bare skin. Don't drench yourself in it. Also, stick to 30%-40% concentration, stronger than that doesn't afford any improved protection.
Insulation - Winter? Summer? Desert? Mountains? Water crossings? All of these considerations will determine what kind of extra insulation you might need during an unexpected overnight stay outdoors.
Illumination - A small keyring-sized flashlight will suffice. Some folks carry a headlamp instead, which is basically a flashlight attached to an elastic band that fits around your head, leaving your hands free.
First Aid Supplies - Don't think that you have to carry enough to do major surgery. A few bandaids, a couple of single use packets of antiseptic cream, and a small bottle of hand sanitizer is a good start. Add tweezers if ticks may be encountered, a needle to drain blisters, plus a piece of moleskin to cushion 'hot-spots' on your feet to keep them from becoming blisters in the first place. I've also found it useful to carry a card that has my blood type and any medicine allergies, plus emergency contact phone numbers. My entire first aid kit fits into a snack-sized ziplock baggie.
Fire Making Materials - Whether you carry matches, a lighter or firesteel, you must practice at home. The last place to learn how well your emergency gear works is, well, in an emergency! Making a spark or flame is only half the battle too. Carry some sort of tinder such as jute twine, dryer lint, or char cloth. Whatever helps you get a fire started when you really need it. Hint: remember that bottle of hand sanitizer in your first aid kit? A dab of that will burn like crazy. I carry a few cotton balls smeared with vaseline. Tease a bit out to a thin, gauzy sheet and when it lights you'll find that it'll burn for 10 minutes or more.
Repair Kit and Tools - I've carried a medium-sized Swiss Army Knife every day for years. Sharp blade, screwdriver, can opener, scissors, tweezers, etc, at one time or another I've been glad to have each and every one. A multitool can be handy to have, but between my knife and about six feet of duct tape, I've never missed not having my heavy Leatherman along on a hike. Besides the duct tape, consider carrying a couple of binder clips, rubber bands and zip ties. A length of dental floss is stronger than ordinary thread, and you already carry a needle in your first aid kit. The idea here is to repair enough to get by, not to make permanent repairs.
Nutrition - You won't starve to death during an unplanned night or two outdoors, but that's not to say that packing an extra meal or snack is without value. In chilly weather, eating something can stoke the body's inner furnace and keep you a bit warmer. Perhaps more importantly, it's hard to overestimate the ability of a simple candy bar to raise your spirits in a difficult situation. I like the traditional GORP (Good Ol' Raisins and Peanuts), which is simply a handful each of raisins, peanuts and M&M's. Infinitely customizable to your tastes, I often add Honey-Nut Cheerios to my mix.
Hydration – Water is critical to survival, much more so than food and this item can be broken down into two related parts: 1. What to carry your water in, and 2. How to make sure that your water is safe to drink.
Carrying options include Nalgene bottles, bladders, or recycled sports drink bottles (which are tough and lightweight, two good traits for gear you have to carry). For a day hike, having 1 or 2 one-liter bottles is enough. I subscribe to the belief that the best place to carry water is inside of me, so I “camel up,” i.e. drink my fill, at water stops and carry an extra liter in my pack.
Treating your water could be the subject of a long post all on its own, so my advice is to do some research on the subject (Google is your friend here) and talk to experienced hikers and campers to see what they do. I’ve known people who successfully used all kinds of different methods, from chemicals to ultraviolet exposure to mechanical filtering. Personally, I use drops called Aqua Mira, it works for me.
Emergency Shelter – for an emergency, I like the “space blanket”, those aluminized mylar sheets that are sold in every camping department. They’re lightweight and take up very little room, just don’t unfold it and expect to get it back to the same size! If you want to see what they’re like spread out, buy a second one. They’re flimsy, think of it as disposable.
So that’s everything on the list, but I’d recommend a couple more items.
Bandana – the Swiss Army Knife of textiles. Mop sweat, filter water, improvised first aid sling, sun protection for your neck, the list is limited only by your imagination.
Emergency whistle – These whistles are louder than your voice, and it's easier to blow than it is to yell at the top of your lungs.
Cell Phone - I always have mine for just-in-case. I always have it turned off.
And the last two essentials would be your brain and experience. The best gear in the world won’t help a bit if you don’t know how to use it. Staying calm in an unexpected situation is easier if you know how deal with it beforehand. Get some first aid training. Find a book on using a map and compass and do some practice work with it. Practice making a fire at home (with adult supervision, of course). Go on some hikes and use your water purification. Get outside and see what works for you and what needs to be improved upon or changed.
By careful and thoughtful selection, it would be possible to fit almost all of this essential gear into an Altoids tin. My “emergency” kit fits into a quart-sized freezer ziplock, and is always with me when I’m outdoors. Have some fun putting together your own essentials kit, and get in the habit of taking it along. I hope that you never need to use it, but it’s comforting to be prepared in case you do.