This chapter originally included some material reprinted from Foulkes’ ‘Introduction to group-analytic psychotherapy’ (1948) but is here somewhat shortened to avoid repetition. His experience with large groups was almost entirely during his army service, at North-field Military Hospital. He subsequently took part in (but did not himself conduct) large groups at workshops and symposia and took great interest in his colleagues’ experiences in this field. He was a keen observer of any large-group activities when visiting hospitals with therapeutic communities.