A succinct account of the genesis and development of the Tavistock and group-analytic models of group psychotherapy focuses on their creators, Bion and Foulkes, and on how their life circumstances and their interpersonal and group attachment histories shaped their thinking and perception of the group and its therapeutic potential. The methodology combines historical investigation and literature review with psychodynamic and group-analytic formulations; it also provides an attachment-based, critical analysis of both approaches, their similarities and differences, and their mutual influence. Likewise, the chapter investigates the evolution of the concept of group attachment, formulated by Bowlby in 1969, which has been largely overlooked in the specialist literature until the last two decades, despite the fact that group lives, as well as interpersonal and group attachment, have played a fundamental role in our survival as a species and in our well-being and healthy development as a person. The present research is also informed by anthropological, psychosocial, organisational, and cultural aspects of human growth. It concludes that group attachment is highly relevant to group psychotherapy and that studying its nature and therapeutic implications should be an integral part of the training of psychotherapists and other mental health professionals, particularly those working with groups.