The etiologic involvement of specific human papillomavirus (HPV) types in cancer of the cervix and their role in a substantial proportion of other anogenital cancers and cancers of the oropharynx (most prominently cancers of the tonsils and of the larynx) label HPV as a prime target for the analysis of mechanisms leading to the development of malignant tumors in humans. As in other viral infections linked to human cancers, HPV infection is not sufficient for the induction of malignant growth. It emerges, however, as the main factor, introducing new genes into the latently infected cells whose function as oncogenes have become clearly established during the past few years. The regulation of their expression by specific host cell proteins, stimulated by intra- and intercellular signals, seems to represent a primary defense mechanism against the induction of unrestricted growth. The failure of this host cell control system, on the other hand, by mutational changes affecting the host cell genome appears to predispose for malignant conversion. This volume summarizes various aspects of HPV research and its relationship to human cancers and provides an overview of current topics in an exciting research field.