Adverse events are undesirable events that can occur during medical or psychological treatment. There has been limited attention to adverse events in psychological treatment trials relative to pharmacotherapy trials. Cognitive behavioural therapy (CBTI) is the first line treatment for insomnia but studies have reported potential negative effects during acute implementation. This review aimed to understand the extent to which adverse events are monitored for and reported in the CBTI trial literature. Ninety-nine randomised controlled trials were identified for inclusion, with findings showing that 32.3% (n = 32) of studies addressed adverse events in some way, while only 7.1% (n = 7) of studies met all criteria for adequate reporting of adverse events. For studies that reported on adverse events by group, there did not appear to be consistent differences between trial arms, however the limited evidence-base coupled with marked heterogeneity in monitoring and reporting makes it difficult to draw clear conclusions at this time. We outline recommendations for the field aimed at improving prospective monitoring and reporting of adverse events in psychological/behavioural treatment trials. Keywords: Adverse events; Cognitive behavioural therapy; Insomnia; Sleep.