At first glance, the question of whether or not psychotherapy works does not seem overly difficult. Yet, considering that psychotherapy, as described in chapter 10, is a dynamic, multicomponent process in which client and therapist influence one another in order to achieve a lasting therapy benefit, one can see that this global question harbors a potential myriad of subquestions. In order to do this complexity justice, this chapter will begin with a brief rereview of the methods that are available for answering questions about therapy effects. Once equipped with tools to understand the pertinent research literature, this chapter begins with a historical review of major developments in psychotherapy outcome research. We will show how outcome research has become much more sophisticated in terms of the questions it asks and also the techniques it uses to obtain responses. Next, actual research findings will be summarized while highlighting the remaining gaps in our knowledge.