What makes therapy work? A multifaceted question.

The question "what makes therapy work?" has many facets. For instance, if there is evidence for a certain form of psychotherapy, one must question whether the positive effects are due to its methods or rather to "common factors" that may be shared with a number of other procedures. Second, because all evidence-based forms of psychotherapy represent treatment packages that involve a large number of components, there is the issue of the relative importance of these various treatment components. A third issue involves mechanisms of change: by what kind of mechanisms do the active treatment components obtain their effects? Finally, a fourth issue concerns the concepts used in these descriptions and analyses: are these concepts sufficiently well defined to formulate good hypotheses about what makes therapy work? In this article, these questions are discussed briefly, while relating them to the eight articles that comprise this special issue.