Bion hardly ever mentions the concept of regression in the classical sense. However, he uses it to understand whether the direction taken by the therapeutic relationship in the here and now of the session is towards psychic growth or rather in the opposite direction. Therefore, asking oneself what it represents for Bion implies inscribing the concept of regression in the framework of the grid, the kind of conceptual compass that he has built to map the possible evolutions of the unconscious emotional climate (O) that pervades the session at a given moment. The author then reports an entire session with a psychotic patient and illustrates the use that can be made of this notion, paired with that of progress, in the post-Bionian theory of the analytic field.