Following its inclusion in ICD-11, the addictive use of computer games has now found its way into health care as a recognized new disorder. Other forms of addictive Internet behaviour can now also be coded as diagnoses, at least indirectly. The Outpatient Department for Gambling Addiction in Mainz has a long tradition in the research and treatment of behavioural addictions and has pioneered the development of appropriate interventions. This manual provides scientifically well-based and practical information on diagnosis and treatment and describes a validated form of behavioural therapy treatment in detail. The work is rounded off with case studies and downloadable materials for treatment.