BACKGROUND: Epilepsy affects about 50 million people worldwide, nearly a quarter of whom have drug-refractory epilepsy. People with drug-refractory epilepsy have increased risks of premature death, injuries, psychosocial dysfunction, and a reduced quality of life. OBJECTIVES: To assess the efficacy and tolerability of clonazepam when used as an add-on therapy for adults and children with refractory focal onset or generalised onset epileptic seizures, when compared with placebo or another antiepileptic agent. SEARCH METHODS: We searched the following databases on 14 September 2017: Cochrane Epilepsy Group Specialized Register, Cochrane Central Register of Controlled Trials (CENTRAL) via the Cochrane Register of Studies Online (CRSO), MEDLINE (Ovid 1946 to 14 September 2017), ClinicalTrials.gov, and the WHO International Clinical Trials Registry Platform (ICTRP). SELECTION CRITERIA: Double-blind randomised controlled studies of add-on clonazepam in people with refractory focal or generalised onset seizures, with a minimum treatment period of eight weeks. The studies could be of parallel or cross-over design. DATA COLLECTION AND ANALYSIS: Two review authors independently selected studies for inclusion, extracted relevant data, and assessed trial quality. We contacted study authors for additional information. MAIN RESULTS: No double-blind randomised controlled trials met the inclusion criteria. AUTHORS’ CONCLUSIONS: There is no evidence from double-blind randomised controlled trials for or against the use of clonazepam as an add-on therapy for adults and children with refractory focal or generalised onset epileptic seizures.