Liquids and gases share the characteristic of variable shape; this is refl ected in their natural tendency to fl ow or change shape. A substance that has a natural tendency to fl ow is called a fl uid (Fuchs 1985). Liquids and gases are both fl uids. Whenever an object moves through a fl uid or a fl uid fl ows over a stationary object, the fl uid exerts pressure on the object; thereby tending to move the object or change the way the object is moving. Fluid mechanics is the study of the forces that act on bodies in fl uids and the eff ects of those forces on the movement of the bodies. Human movement is aff ected by two fl uids, air and water, often simultaneously, as in swimming and sailing. The purpose of this chapter is to describe the fl uid mechanics of air and water in relation to the movement of the human body and projectiles.