Background: Early adolescence (ages 10-14) is a time of rapid and interconnected physical, cognitive and social changes that has special salience for sexual development. While the attitudes and behaviors adopted during these formative years carry lifelong sexual and reproductive health and rights (SRHR) consequences, little is known about gender attitudes and sexual experiences among very young adolescents (VYA). Methods: A mixed-methods systematic review was conducted to synthesize the global peer-reviewed evidence on factors that influence gender attitudes in early adolescence globally. Thematic analysis was used to synthesize findings organized by the ecological framework. A cross-sectional study was conducted to describe the prevalence and patterns in romantic and sexual activities among a representative sample of 11-14-year-olds (in this dissertation I use this age group and VYA interchangeably) in the Korogocho informal (slum) settlement of Nairobi, Kenya (n=365). Latent class analysis (LCA) was used to develop a typology of romantic and sexual experiences. Latent class regression models were conducted to describe the characteristics of the identified subgroups, and subsequently to examine the association between gender attitudes and permissivness towards early heterosexual relationships with romantic and sexual experiences.