This chapter deals with clinical aspects of problematic Internet use. It suggests that formal diagnostic criteria would enhance recognition of and research regarding problematic Internet use. Factor analysis extracted six factors from the Internet Addiction Test (IAT): salience, excessive use, neglect of work, anticipation, lack of control, and neglect of social life. These six factors strongly correlated with each other and showed good to moderate internal consistency. Caplan has described the Generalized Problematic Internet Use Scale (GPIUS), which was reliable and valid in a preliminary study. Patients' Internet use often appears to become problematic soon after they first use the Internet, possibly within 6 months to a year a time when they feel intimidated as well as fascinated by the new technology. Problematic Internet use may respond to serotonin reuptake inhibitors, although larger and longer controlled trials are needed to investigate this possibility.