There is wide agreement that the Internet can serve as a tool that enhances well-being. It is more difficult, however, to find consensus around the issue of problematic Internet use. That may be in part because scientific investigation has lagged far behind technological advances and media attention. The diagnostic schemas that have been proposed since 1996, and the screening tools that have been developed, stress similarities with substance use, impulse control disorders, and obsessive-compulsive disorder. Prevalence figures vary as a function of the diagnostic definition used, the age group studied, and whether the surveys were conducted online. Studies suggest high comorbidity rates with mood disorders and, among younger individuals, attention-deficit/hyperactivity disorder. Treatment should address any comorbid conditions present, as those may be causing, or exacerbating, problematic Internet use. Interventions that may specifically target problematic Internet use include cognitive behavioral therapy and selective serotonin reuptake inhibitors, but detailed guidelines must await further studies. For a medium that has so radically changed how we conduct our lives, the Internet's effects on our psychology remain understudied. More research is needed into the pathophysiology, epidemiology, natural course, and treatment of problematic Internet use. In addition, the more subtle psychological changes, such as disinhibition, that seem to characterize people's online behavior also deserve attention, even if they cannot be seen as necessarily pathological.