Wilfred Bion's seminal work with treatment and training groups is considered by many group therapists to be a classic study of group behavior. Despite the influence of his early work on groups, practitioners frequently find his later writings difficult to apply to group settings. The purpose of this paper is to illustrate the potential usefulness of Bion's later psychoanalytic writings to our understanding of the complex dynamics active in therapy groups. This paper examines the relationship among four concepts that Bion emphasized in his later writings: projective identification, containercontained, (Ps<---->D), and catastrophic change. Two clinical vignettes are presented to demonstrate how these concepts can aid the therapist in understanding the complicated dynamics active in therapeutic group settings.