Internet gaming disorder (IGD) has emerged as a complex and multifactorial behavioral condition that affects individuals worldwide, particularly adolescents and young adults. This chapter provides a comprehensive overview of IGD, including its diagnostic criteria, epidemiology, psychological mechanisms, risk and protective factors, and evidence-based prevention and treatment strategies. We examine key theoretical models—such as cognitive–behavioral and motivational frameworks—as well as game design features that contribute to excessive gaming behaviors. The role of individual traits (e.g., impulsivity, emotional dysregulation), family environment, and social context is discussed through an integrative biopsychosocial model. The chapter also outlines validated screening tools and highlights the need for nuanced assessment to differentiate between high engagement and clinical impairment. Finally, we present preventive approaches and policy recommendations, emphasizing the importance of multisectoral collaboration to promote digital well-being.