Using data from the Nairobi Urban Health and Demographic Surveillance System (NUHDSS), this paper seeks to understand the characteristics of adolescent girls who are circumcised in Kenya. The paper discusses the determinants of female circumcision in two Nairobi informal settlements. It is based on detailed information collected from young persons aged 12-24 targeting major transition events such as first sex, childbirth, marriage, and circumcision. Out of 4058 adolescents and young people interviewed 2010 were adolescent girls and young women. Out of the total number of interviewees, the 527 girls and young women on whom this paper is based are from ethnic communities that practice circumcision. We used the life-table technique to estimate the median age at circumcision and logistic regression to analyse the relationship between female circumcision and adolescent sexuality, controlling for other characteristics. The type of stay within the Demographic Surveillance Area (DSA), religion, ethnicity, residential location and mother education were associated with being circumcised. Current school attendance was not associated with being circumcised but if one had never attended school then they were more likely to be circumcised. For policy making, it is imperative to explore the nature of social networks within which circumcision decisions are taken and enforced.