The benzodiazepines are the most effective, safest, and most widely used antianxiety drugs. As a class of drugs, there are few major differences between the various benzodiazepine derivatives. The main distinguishing features are different plasma half-lives and the presence or absence of pharmacologically active metabolites. Plasma half-lives vary considerably, from 2 to 3 hours to more than 100 hours. All benzodiazepines are equally effective in the short term management of anxiety and insomnia, and their classification into 'anxiolytics' and 'hypnotics' is not justified. There are numerous other indications for benzodiazepine use, such as muscle spasm in osteoarthritic conditions, and acute alcohol withdrawal, but the benzodiazepines have no antidepressive or analgesic effects. While there is no good evidence for their long term efficacy in the treatment of anxiety and insomnia, the benzodiazepines are more effective and safer than their main predecessors, the barbiturates. Some of the benzodiazepines, particularly those with long plasma half-lives which are commonly used as hypnotics, have a prolonged duration of action and cause marked 'hang-over' effects. Alcohol enhances the effects of these drugs, and thus can also increase their side effects. Adversely effects such as oversedation, tremor, ataxia and confusion are much more common in elderly patients. Ever since the benzodiazepines were first marketed 20 years ago their use has increased rapidly, and it is now estimated that between 12 and 16% of the adult population in developed countries use tranquillisers at some time each year. However, their overall use has probably diminished somewhat in the last few years. Although their indications are very common, it is possible that some of this extensive usage may be the result of dependence. Until recently, published reports of such dependence were comparatively few. However, withdrawal symptoms have now been demonstrated in a substantial proportion of patients on long term, normal dose benzodiazepine treatment. The abstinence syndrome usually lasts for 8 to 10 days, and is characterised by insomnia, anxiety, loss of appetite and bodyweight, tremor, perspiration, and a host of perceptual disturbances. More serious developments such as epileptic fits and psychosis are probably infrequent during withdrawal from therapeutic doses. The overall incidence of benzodiazepine dependence remains unknown.