In this era of insistence on evidence-based treatments, cognitive behavioral therapy (CBT) has emerged as a highly preferred choice for a spectrum of psychological disorders. Yet, it is by no means immune to some of the vagaries of client participation. Special concerns arise when clients drop out from treatment. Objective: The aim of this study was to answer questions about the rate and timing of dropout from CBT, with specific reference to pretreatment versus during treatment phases. Also explored were several moderators of dropout. Method: A meta-analysis was performed on dropout data from 115 primary empirical studies involving 20,995 participants receiving CBT for a range of mental health disorders. Results: Average weighted dropout rate was 15.9% at pretreatment, and 26.2% during treatment. Dropout was significantly associated with (a) diagnosis, with depression having the highest attrition rate; (b) format of treatment delivery, with e-therapy having the highest rates; (c) treatment setting, with fewer inpatient than outpatient dropouts; and (d) number of sessions, with treatment starters showing significantly reduced dropout as number of sessions increased. Dropout was not significantly associated with client type (adults or adolescents), therapist licensure status, study design (randomized control trial [RCT] vs. non-RCT), or publication recency. Conclusions: Findings are interpreted with reference to other reviews. Possible clinical applications include careful choice and supplementing of treatment setting/delivery according to the diagnosis, and use of preparatory strategies. Suggestions for future research include standardization of operational definitions of dropout, specification of timing of dropout, and exploration of additional moderator variables.