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Background: Human papillomavirus (HPV) is primarily transmitted by unprotected sexual behaviours, and persistence of this virus is associated with HPV-related cancers of the cervix, anus, vulvar, oral cavity and penis. Nigeria has one of the highest burdens of cervical cancer, but there is a paucity of data on other HPV-related cancers. Research objectives: To describe the epidemiology of HPV and the pattern of genital, oral and anal sexual intercourse and their association with the prevalence of genital, oral and anal HPV infections among adolescents and young adults and female sex workers (FSWs) (18-45 years) in Nigeria. Methods: First, a systematic review of reported oral and anal sex among heterosexual adolescents and adults in sub-Saharan Africa (SSA) was conducted. Second, a qualitative study that enrolled adolescent girls and young women in the community and in brothel-based FSW in Ibadan, Nigeria was conducted. The study used focus group discussions and in-depth interviews to collect data on definitions or meanings, attitudes and interpretations of different sexual behaviours. Third, a cross-sectional survey was conducted in Ibadan amongst girls and young women and FSWs to measure the prevalence and risk factors of oral, vulvar, cervical and anal HPV infections. Participants were interviewed, had a clinical examination and samples were collected for HPV genotyping. Blood samples were also collected for human immunodeficiency virus testing. Results: The systematic review showed that oral and anal sex are commonly practiced in SSA, particularly among adolescents, young adults and FSWs. In the qualitative study, most participants had heard of oral and anal sex; more adolescents and FSWs had heard of these practices than adults. Local terminology/slang terms for oral and anal sex were acceptable to adolescents and FSWs but adults in the community often disapproved of these terms for framing interview questions. The most common motivations for practicing oral and anal sex were protection of sexual relationships and financial benefits. Concerns about these practices included fear of acquiring diseases and physical injury. For the prevalence survey, 310 women, and 315 FSWs participated. Ever having practised oral sex was three times more frequently reported by FSWs than general population girls and women. Only one woman in the general population and eight FSWs had ever engaged in anal sex. The prevalences of any HPV genotypes were higher in FSWs than in general population 4 girls and women in the vulva (88% versus 69%), cervix (84% versus 60%), anal cavity (75% versus 57%) and oral cavity (24% versus 16%). The prevalence of HPV was significantly higher in adolescents and young adults than in adults in the cervix, vulvar and anal cavity in the two surveys. In the adjusted model, there were higher odds of cervical HPV infection among women with concomitant vulvar (Adjusted odds ratio (aOR)= 12.85, 95% CI, 5.70-28.99), oral (aOR= 4.37, 95% CI, 1.50-12.71) and anal (aOR=3.48, 95%CI, 1.74-6.96) HPV infection while women with concomitant cervical (aOR=22.19, 95 %CI 7.85-62.72) and anal (aOR=6.68, 95% CI 2.44-18.26) HPV infections had higher odds of vulvar HPV infection. The odds of having anal HPV infection was associated with history of concomitant cervical (aOR=4.10, 95% CI, 1.85- 9.11) and vulvar (aOR=5.47, 95% CI, 2.11-14.20) HPV infection. Only the history of concomitant cervical HPV (aOR=4.81, 95% CI, 1.58-14.62) was associated with a higher odd of oral HPV. Among the FSWs, history of concomitant vulvar HPV infection was associated with higher odds of cervical (aOR=16.60, 95% CI, 5.08-47.54) and anal (aOR=10.55, 95% CI, 3.67- 30.31 HPV infection, whereas history of concomitant cervical (aOR=6.48, 95% CI, 2.70-15.57) and anal (aOR=8.88, 95% CI, 3.66-23.28) HPV was associated with higher odds of vulvar HPV infections. The socio-demographic, behavioural and other biological factors that were associated with any HPV infections in specific anatomic sites are presented in the relevant chapters. Conclusion: Adolescents and adults, including FSWs in Ibadan, frequently engaged in different sexual behaviours, which might predispose them to sexually transmitted infections. HPV infection was found to be common among sexually active girls and women, with a higher prevalence among FSWs.