In this thesis we use statistical and mathematical models to study different sexual behaviours of relevance for the spread of sexually transmitted infections (STIs). Two groups of special interest for STI interventions are considered: sexually active youths and men who have sex with men (MSM). The focus of the first paper is on mechanisms giving rise to the observed condom use and anal sex behaviour among young people in Sweden. We study the sexual dispositions of individuals, how these interact and generate the observed sexual outcome. The second paper concerns the sexual behaviour of MSM in Sweden and the transmission process of HIV within this population. The population is modelled by a stochastic dynamic network which incorporates both steady partnerships and casual contacts. We model the spread of an infection where individuals are susceptible, infectious, or diagnosed (and unable to transmit) and derive the basic reproduction number R0, the probability of a major outbreak and the endemic prevalence.