The aims of the thesis were to study relationships between the effects of online gaming and gambling and negative social consequences and ill health among adolescents and to determine whether gaming and gambling activities occur together. The papers in this thesis used epidemiological methods to obtain self-report information from Swedish adolescents aged 13–18 years. Time spent in online gaming was associated with negative social consequences, and this relationship was explained by online gaming motives. Gaming for fun and social motives was associated with a reduced risk of negative social consequences, whereas gaming to escape problems, gain status, or meet demands from others was associated with an increased risk. Increased online gaming time on weekdays increased the probability of having depressive, musculoskeletal, or psychosomatic symptoms, and was related to online gaming motives. The probability of ill health was low in those who reported gaming for fun or social motives. Adolescents with symptoms of attention deficit hyperactivity disorder (ADHD) were more sensitive to gambling frequency and to developing a gambling problem. However, among those identified as susceptible, adolescents with ADHD were equally affected compared with other susceptible participants in terms of their gambling frequency. Boys had a higher probability than girls of participating in online gambling in association with online gaming. Having at least one parent born outside Scandinavia was associated with a higher probability of online gambling, especially among girls. The effect of alcohol use as a factor contributing to online gambling was greater among boys than among girls. The results of this thesis contribute new knowledge about sex differences in online gaming and gambling behaviours and add to the limited research on online gaming and online gambling behaviours among adolescent girls. Gaming motives may be helpful for identifying online gamers needing support to reduce their unhealthy gaming behaviour. Information about factors related to gaming and gambling problems may be of interest to clinicians in psychiatry, psychology and social work, as well as to policymakers, parents and teachers involved in adolescent health and development. Effect preventive strategies should consider the sex differences in gaming and gambling behaviour in adolescents.