The Official Rules of Golf appear on the About.com Golf site courtesy of the USGA, are used with permission, and may not be reprinted without the permission of the USGA.

The player may deem his ball unplayable at any place on the course except when the ball is in a water hazard. The player is the sole judge as to whether his ball is unplayable.

If the player deems his ball to be unplayable, he must, under penalty of one stroke:

  a. Play a ball as nearly as possible at the spot from which the original ball was last played (see Rule 20-5), or
  b. Drop a ball behind the point where the ball lay, keeping that point directly between the hole and the spot on which the ball is dropped, with no limit to how far behind that point the ball may be dropped; or
  c. Drop a ball within two club-lengths of the spot where the ball lay, but not nearer the hole.

If the unplayable ball is in a bunker, the player may proceed under Clause a, b or c. If he elects to proceed under Clause b or c, a ball must be dropped in the bunker.

The ball may be lifted and cleaned when proceeding under this Rule.

PENALTY FOR BREACH OF RULE:
Match play — Loss of hole; Stroke play — Two strokes.

©USGA, used with permission