Obsessive-compulsive disorder (OCD) in childhood and adolescence is an impairing condition, associated with a specific set of distressing symptoms incorporating repetitive, intrusive thoughts (obsessions) and distressing, time-consuming rituals (compulsions). This review considers current knowledge of causes and mechanisms underlying OCD, as well as assessment and treatment. Issues relating to differential diagnosis are summarised, including the challenges of distinguishing OCD from autism spectrum disorders and tic disorders in youth. The recommended treatments, namely cognitive behaviour therapy and serotonin reuptake inhibiting/selective serotonin reuptake inhibitor medications, are outlined along with the existing evidence-based and factors associated with treatment resistance. Finally, novel clinical developments that are emerging in the field and future directions for research are discussed.