This article discusses information extracted from 53 studies that have measured adherence to cognitive behavior therapy for insomnia. There has been an increase in more complex and less biased methods for assessing adherence that move beyond simply asking the patients whether they have adhered to the intervention or not. There is a need for a consensus around how to measure adherence, if clinicians want to arrive at an estimate of optimal adherence. Heterogeneity of studies, particularly in the way adherence is operationalized, prohibited conclusions about the relationship between adherence and outcome, as well as about predictors of adherence.