The chapter discusses the development and psychometric properties of the Compulsive Internet Use Scale (CIUS) by Meerkerk et al. (2009). The CIUS is designed to assess the severity of compulsive internet use, a phenomenon where individuals become overly attached to certain online activities, leading to psychological, social, and professional dysfunction. The study aims to address several key questions: what constitutes CIU, how it can be measured, its prevalence, and its relationship with various internet functions and psychosocial well-being. The CIUS consists of 14 items rated on a 5-point Likert scale, covering symptoms such as loss of control, preoccupation, withdrawal symptoms, coping, and conflict. The scale was tested in three studies: two longitudinal studies with representative samples of heavy internet users and one large convenience sample. The results showed that the CIUS had good factorial stability, high internal consistency, and valid correlations with concurrent and criterion variables, including the Online Cognition Scale (OCS). The first study validated the CIUS against the OCS, finding high correlations between the CIUS and OCS subscales related to diminished impulse control. The second study replicated the findings and tested the stability of the factor model over time, confirming its reliability. The third study extended the validation to a large convenience sample, further confirming the scale's psychometric properties. Overall, the CIUS is presented as a reliable and valid instrument for assessing the severity of compulsive internet use, which can be used to better understand and address the issues associated with this phenomenon.The chapter discusses the development and psychometric properties of the Compulsive Internet Use Scale (CIUS) by Meerkerk et al. (2009). The CIUS is designed to assess the severity of compulsive internet use, a phenomenon where individuals become overly attached to certain online activities, leading to psychological, social, and professional dysfunction. The study aims to address several key questions: what constitutes CIU, how it can be measured, its prevalence, and its relationship with various internet functions and psychosocial well-being. The CIUS consists of 14 items rated on a 5-point Likert scale, covering symptoms such as loss of control, preoccupation, withdrawal symptoms, coping, and conflict. The scale was tested in three studies: two longitudinal studies with representative samples of heavy internet users and one large convenience sample. The results showed that the CIUS had good factorial stability, high internal consistency, and valid correlations with concurrent and criterion variables, including the Online Cognition Scale (OCS). The first study validated the CIUS against the OCS, finding high correlations between the CIUS and OCS subscales related to diminished impulse control. The second study replicated the findings and tested the stability of the factor model over time, confirming its reliability. The third study extended the validation to a large convenience sample, further confirming the scale's psychometric properties. Overall, the CIUS is presented as a reliable and valid instrument for assessing the severity of compulsive internet use, which can be used to better understand and address the issues associated with this phenomenon.