This paper reviews the main findings concerning the problems associated with shift work. A conceptual model is presented in which these problems are classified into 3 main interrelated domains, medical, biological and social. The biological problems are evidenced in disturbed rhythms and sleep, the medical problems in improved objective and, in particular, subjective health, and the social problems in impoverished family and social life. While some of these problems are argued to be relatively avoidable, others are considered to be more basic and to stem from the disturbances of circadian rhythms and the sleep/wake cycle. Three main types of intervention are discussed that might alleviate the shift workers' problems. These centre on the type of shift system, the scheduling of zeitgebers or synchronizers, and the counselling of individuals.