This article reviews studies on the effects of weight cycling and weight variability on metabolism, psychological status, morbidity, and mortality. Repeated bouts of weight loss and regain, known as weight cycling or yo-yo dieting, are highly prevalent, occur in males and females, and are common in both overweight and nonoverweight individuals. While there has been no consistent demonstration that, as was first thought, weight cycling makes subsequent weight loss more difficult or regain more rapid, it is possible that this does occur under some conditions or in particular individuals. There are stronger and more consistent links between body weight variability and negative health outcomes, particularly all-cause mortality and mortality from coronary heart disease. Weight cycling may also have negative psychological and behavioral consequences; studies have reported increased risk for psychopathology, life dissatisfaction, and binge eating. The bulk of epidemiologic research shows an association of weight variability with morbidity and mortality, although the mechanisms are not clear at present. There is a clear need for further research on the effects of weight cycling on behavior, metabolism, and health. Understanding and promoting weight maintenance is an important priority.