The comorbidity of obsessive–compulsive symptoms (OCS) in the context of schizophrenia is often not recognised by clinicians, and patients may not report these symptoms until they become severe. However, there is a reported prevalence of 10–24% for obsessive–compulsive disorder (OCD) in schizophrenia and related disorders. The onset of OCS/OCD has been noted to occur both before and after the diagnosis of schizophrenia or schizoaffective disorder. It has also been known to occur following commencement of treatment with antipsychotic medications, especially clozapine. Current literature provides limited guidance for treatment. Review of the current evidence supports: addition of selective serotonin reuptake inhibitors (SSRIs) to antipsychotics; addition of aripiprazole, amisulpride or lamotrigine; or reduction in the dosage of clozapine. There is also evidence supporting the addition of cognitive–behavioural therapy and electroconvulsive therapy (ECT). The SSRIs that are evidenced to be useful are fluvoxamine, escitalopram, sertraline and paroxetine. More studies are needed to expand the evidence base. Early targeted interventions are recommended.