The present study aimed to develop a short, easily administered, psychometrically sound, and valid instrument to assess the severity of compulsive Internet use. A set of criteria was determined based on the addiction literature. Next, the internal consistency and convergent validity were determined, and the set was tested as a one-factor solution in two representative samples of heavy Internet users (n = 447 and n = 229) and in one large convenience sample of regular Internet users (n = 16,925). In these three studies, respondents were asked about their online behavior and about problems related to Internet use. In the first study, the Online Cognition Scale (OCS) was included to determine concurrent validity. The newly developed Compulsive Internet Use Scale (CIUS) contains 14 items ratable on a 5-point Likert scale. The instrument showed good factorial stability across time and across different samples and subsamples. The internal consistency is high, and high correlations with concurrent and criterion variables demonstrate good validity.