In this chapter we consider therapist variables in psychotherapy and psychotherapy research. Our endeavor is not at all a new one, since the topic has been of interest from the very beginning of systematic thought about the complex, dynamic set of processes that define psychotherapy. It is a difficult endeavor, however, because of the massive literature, including expert opinion and systematic research, that has addressed the topic. Fortunately, in the last decade or so two major critical surveys (Meltzhoff & Kornreich, 1970; Parloff, Waskow, & Wolfe, 1978) as well as numerous specialized reviews (e.g., Dent, 1978; Kilman, Scovern, & Moreault, 1979) have systematized the literature either generally, to a specified point in time, or with regard to a particular issue or perspective. Our strategy will be to summarize briefly the conclusions that previous reviewers reached and then, in the process of updating the literature, to comment on selected issues that we view as enduring or promising. One departure from the approach that other reviewers have taken will be our use of the term psychotherapist to include persons, regardless of professional discipline, who synthesize pharmacological and psychological treatment of emotional or behavioral disorders. Also, we will use the terms psychotherapy and psychotherapist in a generic sense to include a wide range of intervention techniques or strategies and persons who practice them. Parloff et al. (1978) critically surveyed the literature on therapist variables through about 1977 and systematically considered most of what had been covered in the earlier major review by Meltzhoff and Kornreich (1970). We consider both of these reviews required reading for the serious researcher, therapist, or planner, since they often critique studies from somewhat different methodological or philosophical perspectives. However, here we will draw mainly on Parloff et al. (1978), since it is the most recent and, in our opinion, more readily accessible survey.