Anxiety and depression are emotional disorders. What are the commonalities, especially in treatment? This question is brought to the forefront by David Barlow and his colleagues in their transdiagnostic approach to anxiety and depression. For over 40 years, a significant part of psychotherapy research has focused on developing specific approaches for particular disorders. However, with the success of disorder-specific approaches, their limitations have also become evident. The level of competence that therapists are trained and supervised to achieve in efficacy studies for specialized therapy approaches typically allows them to only reach and maintain expertise in one or two approaches. Most therapists, however, cannot and do not want to specialize in just one or two disorders, as comorbidities are more the rule than the exception.