The objective of this introductory article is not to review current experimental work in the fields covered by the concept of immunological surveillance. Rather it is to provide a background of historical and theoretical aspects which may be found a useful introduction to the detailed analyses of experimental and clinical data in the articles which follow it. After a brief account to the origin of the concept, I shall try to relate it to current immunological theory and to the far less clearly formulated opinions on the nature of carcinogenesis. Specific tumour immunity must of necessity be highly relevant to the nature of the process by which a cell becomes malignant and I shall include some current speculation on the topic. This is influenced greatly by recent discussions on the evolution of immunoglobulins and the nature of the processes in the genome by which diversity of immune pattern is generated in the individual.