Human papillomaviruses (HPV) are a large group of DNA viruses, with over 60 types identified to date, which can cause the development of benign tumors in the skin and mucosal squamous epithelia. Most of these tumors regress spontaneously but some, especially in the mucosal membranes, become malignant. HPV types with a high risk for inducing malignancies (e.g. 16 and 18) are the subject of increasing interest. HPVs are both host-specific and tissue-specific: some types preferentially infect specific epithelia, giving rise to lesions with distinct topographic characteristics. HPVs are difficult to study because they do not replicate in available in vitro models. In vivo, HPVs replicate well in epithelial cells undergoing terminal differentiation, e.g. in keratinized cells. Some 40 different types have been reported in epidermal keratinocytes, the most common being types 1 and 2 which produce large amounts of viral antigens and viral particles. In contrast, HPVs replicate poorly in the weakly keratinized squamous epithelia which line the digestive, respiratory, and genital tracts. Junctional epithelia, e.g. on the uterine cervix, are especially prone to HPV infection. The most prevalent HPV types in benign genital lesions are types 6 and 11, whose characteristic features include extrachromosomal DNA and production of only small amounts of viral antigens. The profound nuclear and cytoplasmic changes induced by HPVs lead to the formation of koïlocytes which are found mainly in the granular layer of epithelia and have been especially well described in the uterine cervix and vagina. HPV epithelial tumors are squamous cell carcinomas that often harbor HPV types 16 and 18; this is especially true of cervical intraepithelial neoplasias. These tumors contain the viral DNA, which may or may not be integrated into the cellular DNA, whereas viral antigens are lacking. The high incidence and broad spectrum of HPV types found in patients with acquired immunodeficiencies (e.g. under immunosuppressive therapy) suggest a key role for cellular immunity in the development of HPV-induced lesions. A number of cofactors, including UV radiations and smoking, as well as oncogene activation and anti-oncogene inactivation, may increase the risk of progression to malignancy.