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Evidence shows that the efficacy of psychotherapy is limited. Modular psychotherapy has been suggested as a promising approach, aiming at taking inter-individual patient differences and differential treatment effects into account. In modular psychotherapy, therapists are provided with a toolbox of different treatment modules that may be flexibly applied depending on the patient's needs. In this comment, empirical and conceptual issues of modular psychotherapy are critically discussed, using a recent study as an example. In summary, there is presently no evidence that modular psychotherapy is superior to non-modular psychotherapy in adults. In addition, serious conceptual questions arise especially for combining interventions of different theoretical approaches. Alternative research approaches may be more promising, for example, examining the needs of non-responders and dropouts as well as studying doseeffectiveness relationships, that is the question what patients benefit from short-term treatments and for what patients longer-term psychotherapy of what duration is required.