Black bears and grizzly bears are very different animals, with grizzlies being responsible for most bear-attack fatalities.
In the extreme case that a grizzly bear makes contact with you, play dead. Lie face down on the ground and place your hands around the back of your neck. Stay silent and don't move. Try to keep your legs spread apart to prevent the bear from rolling you over. If possible, leave your pack on to protect your back. Typically a grizzly bear will break off its attack once it feels the threat has been eliminated. Remain quiet and motionless for as long as possible. Bears will often watch from a distance and return at the first sign of movement.
Black bears attack very rarely, but when they do it is most likely a predatory attack (i.e. looking for a meal). Even though a normal black bear does not view people as food, a starving or injured bear might. Playing dead or climbing a tree will not stop these kind of attacks, so your best recourse is to act aggressively and try to intimidate the bear by yelling and waving your arms and if necessary, fight back using any object available.
When the bear no longer feels threatened, it will usually leave the area. Do not move or make noise until you are sure the bear is gone.
For further information:
American Grizzly Bear - information provided by the National Wildlife Federation.
The Bear Den - includes lots of good information on bears.
Bears.org - includes information on bear species, as well as bear myths, essays, and references in the media.
International Association for Bear Research and Management - a non-profit volunteer organization dedicated to the conservation of all species of bears.
North American Bear Center - provides educational facilities, exhibits, multi-media presentations to promote an understanding and appreciation of black bears, their role in the ecosystem, and their relationship to humans.
