The questions that guide psychotherapy process research are: what changes in psychotherapy? And, how does that change occur? These questions have been present in clinical settings and psychotherapy research for decades, generating a substantial amount of knowledge for the discipline. Based on a questioning of the limitations of traditional psychotherapy research, whose primary focus has been the study of final outcomes of treatment, this area of research has exposed important conceptual and methodological proposals for the study of the process. Despite its long and productive tradition, research into the psychotherapeutic process is currently gathering a new momentum from the growing conviction of the need to focus efforts on studies that can account for the elements, mechanisms and processes associated with change. In this introduction we organize the field of psychotherapy process research, according to its units of analysis — macro- or micro-processes — and the foci on which they are based: content, factors and mechanisms of change. The different ways in which these components are organized are exemplified with the articles included in this monograph. We then give an overview of the most used methods in this field of study, also referring to the studies that report on the application of these methods in this special issue of Studies in Psychology.