The Northfield Experiments took place at Hollymoor Hospital, Northfield, Birmingham, during World War II. The first experiment was conducted by Bion & Rickman. The second evolved gradually; many people contributed to its success, including Foulkes, Main and Bridger. The experiments were an important landmark in the evolution of theory and practice in group psychotherapy and in the therapeutic community movement. They were not carried out solely as responses to the need for mass treatment of neurotic disorders among army personnel; antecedent factors, the theoretical orientation of the practitioners and the nature of army life were equally important. The two experiments differed in pace and in recognition of the needs of higher-order systems, particularly the military hierarchy. They shared many underlying concepts, including responsibility to society, the therapeutic use of groups (including the hospital community) and an emphasis on process. Lessons learned at Northfield remain relevant to the practice of psychiatry today.