Rule of Threes

The first step in emergency preparedness is knowing where to start. What is most important?
Fortunately this is answered by the Survival Rule of Threes:

An average person can survive:

3 minutes without air

3 hours without shelter in harsh conditions of cold or heat

3 days without water

3 weeks without food

This is best illustrated by a story. A friend of a friend died in his car during a blizzard. He had pulled over to the side to avoid being hit or causing an accident and kept the car running to keep warm. However the snow piled up and buried the exhaust pipe causing the car to fill with Carbon Monoxide fumes - a silent killer. This person forgot that fresh air trumps heat for survival. This is also one of the reasons you see warnings not to use cooking stoves inside a tent.

A second example: if you're lost in the wilderness with dry food but no water - don't eat the food! Digesting food uses up moisture inside the body. It is better to go hungry than to become dehydrated.

Here are two other sites discussing the Rule of Threes:
Outdoor Skills
Survival Topics