Obsessive-compulsive disorder (OCD) is estimated to affect between 0.5% and 2% of children and adolescents. The majority of clinical cases has both obsessions and compulsions. Depression, anxiety and tic disorders are common comorbid diagnoses, and assessment can therefore be complex. Several theories have been proposed for the aetiology and maintenance of the disorder, including cognitive-behavioural and neuropsychiatric theories. Cognitive-behavioural and pharmacological treatments are both probably efficacious therapies for children and adolescents but further research is needed to compare these treatments with each other, and to examine the use of combination therapies in controlled trials.