Epidemic Models of the Onset of Social Activities (EMOSA models) describe the spread of adolescent transition behaviors (e.g., sexuality, smoking, and drinking) through an interacting adolescent network. A theory of social contagion is defined to explain how social influence affects sexual development. Contacts within a network can, with some transition rate or probability, result in an increase in level of sexual experience. Five stages of sexual development are posited. One submodel proposes a systematic progression through these stages; a competing submodel treats each as an independent process. These models are represented in sets of dynamically interacting recursive equations, which are fit to empirical prevalence data to estimate parameters. Model adjustments are substantively interpretable and can be used to test for and better understand social interaction processes that affect adolescent sexual behavior.