In recent years, there has been a renewed interest in obsessive-compulsive disorder (OCD) among psychiatrists due to several factors. OCD is not a rare disease as was believed in former times. New epidemiological studies demonstrate that 1–2% of the adult population suffer from OCD. This result was confirmed by several epidemiological studies among different cultural environments which are summarised by Dr Bebbington in this supplement. Thus, OCD figures among the most common of mental disorders. Lately a unique category of disorders has emerged that share some common key features with OCD. These ‘obsessive-compulsive spectrum disorders’ and their treatments are characterised in Dr Hollander's paper.