Knowledge of non-Hodgkin's lymphomas has increased enormously in the last 10-15 years. The various types are considered to reflect the normal immunological processes that take place in the lymphoid tissue after antigenic challenge. This concept, which states that non-Hodgkin's lymphomas are neoplastic counterparts of normal processes, has implications for morphology, immunology and clinical behaviour. Based on this concept, as well as on immunohistological and functional data, a hypothetical scheme of B-cell development is proposed. The relation to normal physiology also holds for lymphomas localized outside the lymph nodes. Normal structure and function in two important extranodal lymphoid tissues, the gastrointestinal mucosa and the skin-associated lymphoid tissues, are related to the lymphomas arising in these sites. This relation and some of its implications are discussed, with special reference to the important processes of homing and recirculation of lymphocytes.