The crunch is the most basic ab exercise. It is arguably the most effective as well. All ab exercises involve some sort of "crunch" motion. The following are instructions for performing the abdominal crunch:

Lie down on your back with your knees bent. Your feet should be flat on the floor. Place your hands in a comfortable position. Generally, the hands are placed either across the chest, behind the head, or extend forward in the direction of your heels (palms flat on the ground). Once you are in the starting position, contract your abdomen by rolling your upper torso forward. Your shoulders, head, and neck should elevate slightly, but should not come too high off the ground. As you crunch, be sure to exhale. Perform this exercise slowly and hold the crunch position for 2 or 3 seconds before returning to the beginning position.

Important: Try to keep your head and neck in line with your spine. Don't crank the neck or let your chin touch your chest.