The growing realisation that the benzodiazepines have potential for causing serious harm has caused concern due to their wide and common use. This paper is a review of the adverse effects of benzodiazepines, and concentrates on four areas of particular concern: drug dependence with the consequent withdrawal symptoms; psychological effects while on the drugs; use by the elderly; and tolerance to the drug effects. Although the phenomenon of a benzodiazepine withdrawal syndrome is generally accepted, there is still controversy over the frequency amongst users. A number of major studies are reviewed here, and the main methodological issues are discussed. These include definition of the withdrawal symptoms, selection of subjects, and use of double-blind, placebo-controlled conditions. The studies investigating psychological impairment with benzodiazepine use deal mainly with motor performance and co-ordination, although there is a large group of studies looking at the effect of the drugs on memory. Although the studies reviewed make a considerable contribution to the understanding of the effects of benzodiazepines, they focus on physiological and specific psychological variables, rather than more global measures of functioning and behaviour. It is suggested here that this emphasis needs to change in order to obtain a clearer picture of how benzodiazepines affect quality of life. Future studies should also be prospective in design, and include clear criteria for the selection of subjects and for the definition of withdrawal symptoms.