Common factors, factors that are common to all forms of psychotherapy, such as the relationship between patient and therapist (alliance), have been much discussed as potential mechanisms that explain how psychotherapy works. Fueled by the dodo bird verdict (the finding that many psychotherapies produce equivalent outcomes), this debate continues to date. Opponents of the common factor theory point to the correlational designs of common factor studies that do not allow causal inferences, whereas the proponents claim that the fact that most psychotherapies produce equivalent results shows that common factors must be responsible for the effects. In reality, there is little evidence that pleads for or against the common factor theory. Recent studies have begun to disentangle the temporal relation between therapeutic alliance and outcome (i.e., what changes first), but the findings are mixed. New lines of research are needed to resolve the debate. The question of which factors are responsible for the benefits of psychotherapy remains an empirical one.