The term “excessive Internet use” is often associated with determining pathological extensive Internet usage, which could be also called “online addiction”. Such excessive presence online is usually defined by the following components used for determining other addictive behaviours: salience, mood change, conflicts, tolerance, relapse, reinstatement and withdrawal symptoms. The described behaviour may lead to a social, mental and also physical impairment of children and youth. In this chapter, we introduce and show the prevalence of five dimensions of excessive Internet use among European children. We also analyse its relation to other psychosocial variables, such as self-efficacy, peer problems, and other kinds of risky behaviour offline and online, i.e. cyberbullying and meeting strangers online.