Background: Sexual behaviour among schoolchildren and adolescents is a sparsely researched area and there are delicate methodological obstacles and ethical concerns when conducting such research. Still it is a subject that engages both parents and professionals. A sexualized behaviour or an early sexual debut (younger than 14 years) can be a sign of sexual abuse. It is therefore of importance to describe what is common and what is uncommon sexual behaviour among children and what the consequences of an early or a late sexual debut may be for the individual upon reaching late adolescence. Adolescents who identify themselves as lesbian, gay or bisexual (LGB) are also a group that needs further attention and research since they are often described as having a lower quality of life and more often experience child abuse than heterosexual teens. Aims: The research leading to this thesis had four goals: 1) to elucidate the sexual behaviour of children between the ages of 7 and 13 as observed by their parents, 2) to investigate the relationship between an early sexual debut (before 14 years of age) and socio-demographic data, sexual experience, health, experience of child abuse and behaviour at 18-years-of-age, 3) to explore associations with no sexual debut (no oral, vaginal or anal sex) at the age of 18, and 4) to describe the relationship between sexual identification and socio-demographic background data, sexual behaviour, health and health behaviour, experiences of child sexual and/or physical abuse and present behaviour among Swedish adolescents. Methods: The parents of 418 children answered questionnaires about their child’s behaviour, both general and sexual, and a sample of 3432 Swedish high school students completed a survey about sexuality, health and abuse at the age of 18. In addition, 362 members of the Swedish Federation for Lesbian, Gay, Bisexual and Transgender Rights (RFSL) completed the same survey at a mean age of 21.4 years. Results: Most of the sexual behaviours among the schoolchildren were common, and in part related to, or varied with, age and gender. A small number of sexual behaviours often referred to as sexualised or problematic behaviours (for example, kissing adults with the tongue, imitating intercourse, masturbating in public, and touching other children’s genitals with the mouth) were found to be very unusual or not reported by any parent in this normative group of Swedish children. Among the adolescents, an early debut (younger than 14 years of age) correlated positively with number of partners, experience of oral and anal sex, smoking, drug and alcohol use and antisocial behaviour, such as being violent, lying, stealing and running away from home. Girls with an early sexual debut had significantly more experience of sexual abuse while boys with an early sexual debut were more likely to have a weak sense of coherence, low self-esteem and poor mental health, together with experience of sexual abuse, selling sex and physical abuse. A multiple logistic regression model showed that a number...