Circuit training is a popular form of workout that targets aerobic fitness and muscular endurance simultaneously. In a circuit training class, participants work through a series of eight to 12 or more exercises for either a set time or number of reps, with little or no rest between exercises. Some circuits may involve the use of exercise equipment; others may use only body weight exercises. Others may combine both. Circuits can be organized in a number of ways to challenge the participants and keep the workout fresh and interesting.