Headtorches are an essential piece of outdoor kit, whether for walking, climbing, mountaineering or even just around the campsite for reading in your tent or finding your way to the toilet at night! The only way to have your hands free to read a map or fiddle with kit is a headtorch.
Headtorches nowadays last for often over 100 hours on AA or AAA batteries. If you have an old Petzl headtorch with a classic bulb and a square battery, frankly just throw it away - there is absolutely no advantage in using them for general purposes unless you like a yellow light and a torch that weighs 3 x as much and lasts only 2 hours...
Emergencies - don't forget that the very concept of emergencies is that ideally they don't happen - having a tiny headtorch in the bottom of your pack is something every outdoor person should do, every time you go out. It doesn't matter how long you think it'll be before you're back, because the idea is that if something happens and you do get stuck out for hours longer than you expected, then you'll have a torch. Of course things happen to others too - I've been caught out when rescuing other people who have come into difficulties and my spare torch was essential to say the least - yet I'd expected to be back well before nightfall.
Which torch is right for me?
Emergencies - try any of the small Petzl torches. The very lightest and tiniest is the E+Lite, but often a Zipka is good as it gives better light and the batteries are the more easily available AAAs.
General use - Tikka Plus is bright, and not much bigger than an emergency light.
Night navigation - only a torch with a big beam will do. For me it's the Petzl Myo.
What does the author use? After 45 years of climbing and mountaineering I keep a Petzl Zipka in the bottom of my pack permanently. My regular torch is a Petzl Myo - once you have a big beam like that there's no going back! The weight of both of these is negligible on most occasions. Sometimes I will take a Petzl Tikka Plus if going very lightweight and there's no night navigation involved.
FULL INFORMATION ABOUT LIGHTING
Petzl have a whole section on their website about lighting and headtorches, how it works, which torch to use, Alkaline vs. Lithium batteries etc - much more comprehensive than we can put here! Go to: http://www.petzl.com/en/outdoor/headlamps/lighting
Items designed to be waterproof have a rating called IPX to say just how waterproof - see below: