Aerobatic flying subjects pilots to gravitational effects (G's) that can impair their ability to safely operate the aircraft, pilots who engage in aerobatics, or those who would take up such activity, should understand G's and some of their physiological effects. This circular provides background information on G's, their effect on the human body, and their role in safe flying. Suggestions are offered for avoiding problems caused by accelerations encountered in aerobatic maneuvers.