To review research relevant to understanding the psychological, social, and biological pathways by which unemployment may affect health risk; to consider the importance of four specific mechanisms; and to indicate some directions for future research. Studies were chosen to illustrate the development of four major hypotheses regarding the relationship between unemployment and ill health, as well as the present state of knowledge. The review therefore includes some much-cited "classics" drawn from a long time span. Where recent reviews already exist relevant to individual mechanisms, these are referred to. Recent (since 1987) reports were sought by searching the BIDS data base. Particular effort was made to locate studies which enabled alternative hypotheses to be evaluated, and to point out where existing evidence is inconsistent or incomplete, indicating the need for further research. To understand the relationship between unemployment and ill health and mortality, four mechanisms need to be considered: the role of relative poverty; social isolation and loss of self esteem; health related behaviour (including that associated with membership of certain types of "subculture"); and the effect that a spell of unemployment has on subsequent employment patterns.