The past four decades have witnessed a transformation in research on the benefits of psychological therapies. However, even though therapists highlight that negative and adverse effects are seen in day-to-day practice, research on the negative effects of psychotherapy is insufficient. Given the unrelenting popularity of therapies, the argument for examining the adverse effects of psychotherapy would seem to be compelling. Such a strategy would extend beyond supervision of individual therapists to the introduction of monitoring systems that allow for a more systematic examination of failed psychotherapy interventions (such as exist for medication prescribing). The starting point could be the development of a consensus on how to define, classify and assess psychotherapy side-effects, unwanted events, adverse reactions, etc. This would provide a conceptual framework for communication, monitoring and research. This approach should not be viewed as an attack on therapies: every branch of medicine learns from mistakes, the same must surely be true for psychological treatments.