Having become an integral part of everyday life, it is not surprising that the Internet has also given birth to a wide range of problematic and, in some cases, pathological behaviours. Under the vague term “Problematic Internet Use” (PIU), many different forms of harmful Internet use are included. A thorough assessment of the most widely reported forms of PIU can help clinicians to recognise core symptom dimensions. Psychopathological elements reminiscent of impulse control, obsessive-compulsive spectrum and substance use disorders have been reported in Internet addicts, with the presence of psychiatric comorbidity representing the rule rather than the exception. Ultimately, Internet addicts can experience significant functional impairment, often reporting negative impact on the family, work and school performances, and legal difficulties. While diagnostic criteria are still investigational, and epidemiologic and treatment studies remain necessary, herein we will provide an overview of the main psychopathological characteristics, clinical phenotypes and scientific advances on PIU.