This systematic review (PROSPERO registration CRD42020158010) aimed to: 1) assess how adherence to cognitive behavioural therapy for insomnia (CBT-I) has been measured; 2) evaluate predictors of adherence; and 3) determine whether treatment outcome is associated with adherence. Inclusion criteria included: adults with insomnia; an intervention of CBT-I, including sleep restriction and/or stimulus control; a reported measure of adherence; and written in English. Searches of eight databases returned 2038 publications as of April 2021. The final sample included 102 papers. Publication quality and risk of bias were assessed using Joanna Briggs institute tools. Studies assessed either global adherence or adherence to specific components of CBT-I via questionnaires, sleep diaries, interviews or actigraphy. Twenty-eight papers examined predictors of adherence. Better pre- and post-session sleep, greater psychosocial support, increased self-efficacy, and fewer dysfunctional beliefs about sleep predicted greater adherence. Twenty-eight papers examined whether adherence predicted treatment outcomes. Only insomnia severity index scores post-treatment were consistently predicted by adherence, and only by a few measures of adherence. Overall, there was very little consistency in how adherence was measured, and in predictors and outcome variables assessed. A standardised method for assessing specific adherence constructs is indicated, to fully understand the role of adherence in CBT-I.