This publication provides an overview of theory and research on Internet addiction. The theory chapters provide a basic introduction to behavioural addictions and Internet addiction. The research chapters study Internet addiction on three different levels: general population level, group level and individual level. The first research chapter contains a quantitative study which establishes the prevalence of Internet addiction in the general population of the Czech Republic and compares some of the results with similar studies in Chile and Sweden. The second research chapter contains a qualitative study which maps the motivation of Internet addiction in the group of MMORPG players and the third chapter provides a case study of the development of Internet addiction and its treatment.