Collusion, a specific transference-countertransference reaction, occurs in psychiatric and medical settings, in couples and within and between groups. The conceptual development of collusion, closely related to the recognition of countertransference, projective identification and the interactional dimensions of psychotherapy, has been inspired by psychoanalytic and systemic theories.

Collusion appears somehow scattered and under different names in the literature and dooms - despite its epistemological value - a somehow shadowy existence.

The aim of this critical narrative review, based on a comprehensive consultation of the literature and informed by our experiences as psychotherapists, supervisors and researchers, is threefold. First, to define and conceptually clarify dyadic collusion, to delineate it from other transference-countertransference experiences and to situate collusion historically and epistemologically; second, to describe in depth and revisit the clinics of dyadic collusion; and third, to classify and discuss different types of dyadic collusion.

Collusion, viewed as a concept, has the potential to bring together different schools of psychotherapeutic thought but also patients and clinicians, the private and the professional, as well as medicine, psychiatry and the social sciences.

A twin article addresses polyadic collusions, discusses contextual determinants of collusion and provides a perspective on teaching and research.