The field of human cancer vaccines is currently undergoing a revival. This volume describes a variety of approaches to active immunotherapy, some traditional and some more technologically sophisticated. Interpretation of their claims to clinical efficacy requires an understanding of some of the principles of tumor immunotherapy. These include the following: (1) human cancers are weakly immunogenic or not immunogenic at all; (2) cell-mediated immune responses, mainly those of T lymphocytes, are critical to tumor rejection; (3) almost nothing is known about the nature of the rejection antigens on human tumors; (4) it is not clear whether these antigens are different for each case of human cancer or whether there are clinically useful antigens that are shared by all or most tumors of a given histologic type; and (5) the effectiveness of vaccines is greatly limited by excessive tumor burden. The reader is invited to approach this collection of papers with a combination of optimism and skepticism.