This article presents the theoretical, clinical, and practical arguments supporting a process‐based transdiagnostic approach to psychotherapy. A working definition of “psychological process” is provided, as well as a tri‐dimensional categorization of psychological processes potentially involved in psychopathology. Guidelines are proposed to select psychological interventions based on the active psychopathological processes evidenced in a given case. We also provide a rationale to organize treatment as a set of modules, each addressing a specific psychopathological process. Next, we review the main processes that may be active in mood and anxiety disorders, and that are accessible to clinicians in regular practice. For each process, we propose a validated assessment questionnaire. Finally, we offer a free‐access web‐based instrument that allows clients to fill in these questionnaires via an internet survey, and that provides therapists with a tool to easily decode and interpret the questionnaire results and to present them to the clients.