Attachment theory, developed by John Bowlby, has been empirically well supported and therefore increasingly been integrated into the clinical and psychotherapeutic field. Meanwhile, there is a wide variety of studies from clinical attachment research. Most of these studies are related to individual psychotherapy. The therapeutic relationship, for example, is often considered as an attachment relationship. Empirically, correlations between the quality of the therapeutic alliance and attachment characteristics of the patient, recently also the therapist, have been investigated. Related to group psychotherapy, attachment theory has been equally considered and discussed in a couple of theoretical papers. Empirically, attachment constructs are only sparsely studied within groups during the recent years. Based upon some principal considerations focussing on the significance of attachment within groups, this article summarizes research results. These relate to the relationship between attachment characteristics and group process within non-clinical groups, the significance of these characteristics for the expectations and attitudes towards groups among patients and therapists within psychotherapy as well as the question if group treatment outcome can be predicted by attachment characteristics. Based upon the results, some aspects of group treatment can be discussed on the basis of attachment theory which might be of relevance for practising clinicians.