Dieting is the practice or habit of eating (and drinking) in a regulated fashion with the aim of losing (or, sometimes, gaining) weight, or in some cases to regulate the amounts of certain nutrients. For example, a person who has diabetes is often on a diet that very carefully regulates blood sugar level.

Certain religions, including Judaism and Islam also impose restrictions on diet, but these are not usually thought of as 'dieting'.

Most typically "dieting" means eating in a carefully planned way to in order to reduce excess body fat. In this sense, there are a number of so-called diets that can be used as part of a regimen of dieting. Also in this sense, excessive dieting is related to anorexia nervosa and bulimia, which are life-threatening conditions. Weight Watchers is a well-known organization that supports weight loss efforts.

Yo-yo dieting, also known as weight cycling, is a repeated loss and gain of body weight due to excessive dieting. The term "yo-yo dieting" was coined by Kelly D. Brownell, Ph.D., at Yale University, in reference to the cyclical up-down motion of a yo-yo. In this process, the dieter is initially successful in the pursuit of weight loss but is unsuccessful in maintaining the loss long-term and begins to gain the weight back. The dieter then seeks to lose the regained weight, and the cycle begins again.

The reasons for yo-yo dieting are varied but often include embarking upon a diet that was initially too extreme. At first the dieter may experience elation at the thought of loss and pride of their rejection of food. Over time, however, the limits imposed by such extreme diets cause effects such as depression or fatigue that make the diet impossible to sustain. The dieter reverts to their old eating habits, and with the added emotional effects begins to rapidly regain weight.

 This kind of diet is associated with extreme food deprivation as a substitute for good diet and exercise techniques. As a result, the dieter may experience loss of both muscle and body fat during the initial weight-loss phase (weight-bearing exercise is required to maintain muscle). After completing the diet, the dieter is likely to experience the body's famine response, leading to rapid weight gain of only fat. This is a dangerous fat-cycle that changes the body's fat to muscle ratio, one of the more important factors in health..