This online resource provides a comprehensive overview of the foundational and functional competencies related to the field of group psychology. It describes the potential treatment benefits of group work and shows how the wide range of applicability makes this a relevant resource across diverse areas, regardless of the population receiving treatment or the theoretic orientation of the therapist. This volume distils the uniqueness and contributions of the specialty in a way that benefits not only psychologists who specialize in group psychotherapy, but also clinicians who have previously taken a more traditionally individual approach to treatment.