Video games, including online ones, constitute very popular interactive entertainment, enjoyed non-problematically by many individuals worldwide. However, uncontrolled gaming and the occurrence of its negative consequences require appropriate diagnosis. Gaming disorder manifests as uncontrolled, persistent, or recurrent behaviour with gaming taking priority, despite its negative impact on personal functioning. Excessive use of the Internet unrelated to gaming, such as excessive use of social media, viewing pornography online, and internet gambling games, is excluded from the concept of disordered gaming. The worldwide prevalence is 3%, rising to 4.6% in adolescents, affecting more males than females. The pathogenesis is heterogeneous, with various psychological contributory factors, such as fascination with an alternative world and selfconcept, low self-esteem, impulsivity, dysregulation of the reward system, and escapism. Individuals with reduced dopaminereceptor function are prone to developing disordered gaming, in particular under stressful life events. Imaging studies have demonstrated changes in brain activation, such as hyperactivation of the anterior cingulate cortex, dorsolateral prefrontal cortex, posterior inferior frontal gyrus, caudate, posterior cingulate cortex, and precuneus; in contrast to hypoactivation of the anterior inferior frontal gyrus, posterior insula, and precentral and postcentral gyri. With respect to psychiatric comorbidities, such as attention deficit hyperactivity disorder or major depressive disorder, several drugs have been effective in treating gaming disorder, namely bupropion, escitalopram, fluoxetine, paroxetine, methylphenidate, and atomoxetine. The most effective psychological intervention was cognitive-behavioural therapy. There is a need to conduct further randomised, double-blind studies controlling for comorbid mental disorders and carried out on larger groups of patients.