This volume is not intended as review of the large literature on tumor antigenicity and efforts at tumor immunotherapy. Its pur pose, rather, is to present discursively an outline of the likely approaches to immunological intervention in neoplastic diseases which present themselves today, in light ofthe probable antigenic properties of cancer cells. References are cited only selectively, in illustration of some of the major considerations to which allusion is made and of some of the supportive evidence. No attempt is made at inclusiveness in the citation of concepts and fmdings. If undue emphasis appears to be given to some aspects of the litera ture and only sparse documentation to others, the grounds do not lie necessarily with a critical estimation of the extent or quality of reported work, but rather with the bias of the writer who consi ders stress on some facets of the field more appropriate than on others for elaboration of his arguments. The references brought in support of a given point are often intentionally varied, including both reports of original work and reviews, very recent observa tions and contributions that gave initial impetus to investigations, in an attempt to exemplify the pertinent literature; and reference is made both to data presented and to concepts advanced."