Clinicians' impact on substance use disorder treatment has been much less studied than therapy and patient variables. Yet, in this selective review of literature, a growing body of empirical work on clinicians' impact highlights several key issues that have relevance both to clinical practice and future research. These issues include clinicians' effect on treatment retention and outcome, professional characteristics, recovery status, adherence to protocols, counter-transference, alliance, personality, beliefs about treatment, and professional practice issues. Specific recommendations are offered to help improve the quality of care clinicians provide. In particular, it is suggested that greater accountability for clinicians' performance be balanced with increased support for their very difficult role. Methodological issues in studying clinicians are also addressed.