Several group models are defined: a mechanistic model, a model of conflicts, a cybernetic model, a field-theoretical model, an organismic model, an interactionistic model. In all these models the group is a dynamic entity which is based on a mutual dependence of the members. From this sociological-horizontal-interactional point of view the group is therefore an entity. From a psychological-vertical aspect the group is rather a common situation in which the individual members remain in their experience separated from each other. To understand the group therapeutic dynamics it is necessary to keep in mind these two levels. From the sociological aspect, e.g. the frequency and kinds of interactions, the relationship of the inside-distances between the members to the distances of the members to the outside is decisive for group cohesion. The more the total interaction in a group augments, the more the emotionally loaded interactions increase. The significance of group cohesion in the therapeutic process is documented also by means of standardized rating scales. The normative effect of the group, the different roles in it, the group structure have to be considered if the group is to be used as a therapeutic mean. From the psychological point of view five phases of group development can be differentiated: 1) explorative contact, 2) regression, 3) catharsis, 4) insight, 5) social learning. By a factor analysis Lieberman et al. have found out that independent of the method used, the following types of group leaders were successful: the "provider", the "social engineer", the "energizer".