This is a great read and good source of information.
Ten Essential Systems
Navigation: map, compass, altimeter, GPS device, personal locator beacon (PLB) or satellite messenger
Headlamp: plus extra batteries
Sun protection: sunglasses, sun-protective clothes and sunscreen
First aid including foot care and insect repellent (as needed)
Knife plus a gear repair kit
Fire matches, lighter, tinder and/or stove
Shelter carried at all times (can be a light emergency bivy)
Extra food Beyond the minimum expectation
Extra water Beyond the minimum expectation
Extra clothes Beyond the minimum expectation
The exact items from each system that you take can be tailored to the trip you’re taking. For example, on a short day hike that’s easy to navigate you might choose to take a map, compass and PLB, but leave your GPS and altimeter behind. On a longer, more complex outing, you might decide you want all those tools to help you find your way. When deciding what to bring, consider factors like weather, difficulty, duration, and distance from help.
Continue reading below for more information about each of the Ten Essential systems. And for help figuring out what else to bring with you, check out these hiking checklists.