I. Introduction Parvoviruses belong to the large group of viral agents of which virologists have become aware by chance in many biological materials due to the availabil­ ity of more sensitive isolation techniques and the extensive use of the electron microscope. In general, many of these viruses lacked the stimulating background of an infectious disease and, therefore, have fallen into oblivion already soon after discovery. In case of parvoviruses, however, interest has been maintained because of the circumstances under which most of them were isolated. A great number of parvoviruses has been recovered from tissues of tumor­ bearing animals, from cell-free filtrates of tumors, or from stable cell lines of tumor origin. These observations necessarily suggested the newly isolated viral agents of playing an important, yet unknown role in the induction and develop­ ment of cancer. On the other hand, further parvoviruses were found constantly associated with adenoviruses. It was the experimental analysis of the multiplica­ tion behaviour which then revealed that the association between parvoviruses and tumors or parvoviruses and adenoviruses originates from the basis of a cer­ tain genetic defectiveness. For some members of the group this may be overcome by cellular helper effects in rapidly growing tissues, for several others, however, by biochemical events in the simultaneously occurring replication of an adeno­ virus only. Additional points of view in favour of parvovirus research have arisen from experimental animal studies.