We have written this book to address people with a profound interest in all approaches to psychotherapy, as well as those interested in a general theory of psychotherapy. We allow two threads of reading, one that is accessible to practitioners and psychotherapists and one that includes mathematical sections and “Info-Boxes,” for advanced psychotherapy researchers and graduate students. In this book, we have developed an encompassing model of the process of psychotherapy, based on the current state-of-the-art psychotherapy research. Therapeutic interventions are staged in the therapist-client relationship and become effective by the interplay of deterministic (“causation”) and stochastic forces (“chance”). This is modeled using the Fokker-Planck equation and by applying principles of complexity theory. Modern theory in psychotherapy is thus complemented by a structural-mathematical framework. Using this framework, we developed statistical tools, which can be applied to empirical time series of psychotherapeutic processes. We provide numerous empirical examples of such applications, expecting that the approach adopted in this work has the potential to advance psychotherapy research and psychotherapy in general.