In recent years there has been an explosion in research on the papillo- maviruses. The viral nature of human warts was first suggested 80 years ago by Ciuffo, who demonstrated transmission using cell-free filtrates. Shope described the first papillomavirus over 50 years ago as the etiologic agent in infectious papillomatosis in rabbits. Subsequent studies by Rous established that benign rabbit papillomas induced by this virus could progress to carcinomas when treated with specific nonviral cofactors. Despite these rich beginnings, the papillomavirus field lay virtually dor- mant until the late 1970s because no one was able to propagate these viruses in culture successfully. In the late 1970s the molecular cloning of the papillomavirus genomes permitted investigators to partially cir- cumvent this obstacle to their progress. The cloning of the viral genomes permitted the standardization of viral reagents and provided sufficient material to begin a systematic evaluation of the biology of this group of viruses.