It is a fundamental problem of groups that they can display behavior different from the behavior of members in isolation. Freud's theory explains this phenomenon beautifully, but his explanation turns out to apply only to a restricted type of group—one consisting in the simple summation of all the members composing it. The problem of heterogeneous groups therefore remains. Bion's theory goes a long way toward mapping the behavior of such groups; his theory may be seen as a general one embodying Freud's as a special case when the members have a common need and focus and a special kind of leader. In addition there is the problem of normality, which for Bion is achieved in the work-group. A brief indication of the relationships of these three types of groups to one another is made. The author does not attempt to provide a theory to explain group bonding.