Benzodiazepines are the most commonly used psychotropic drugs, prescribed for their action as tranquillizers, hypnotics and anti-epileptics. They act in the brain by binding to specific, highly selective recognition sites, the benzodiazepine receptors. This book, first published in 1987, summarises our knowledge of these receptors for a student and professional audience and is divided into two sections. In the first part, the general properties of the receptors and their involvement in the mechanisms of action of benzodiazepines are described. The second part discusses the function of the receptors and concludes that the system represents a novel neuronal system for the fine adjustment of brain function or for auxiliary function if the other neuronal systems fail owing to pathological defects.