Internet and smartphone addiction have become an increasingly significant issue in modern society. It is characterized by excessive and uncontrolled use of these devices, which can severely impair individuals’ well-being and quality of life. Overusing the Internet and smartphones can have a profound impact on personal and interpersonal relationships, professional and academic performance, social life, as well as physical and mental health. Common symptoms of Internet and smartphone addiction include excessive preoccupation with devices, withdrawal symptoms when not using them, a compulsion to extend usage, difficulty in reducing or controlling usage time, and a loss of interest in other activities. Individuals may continue using these devices despite experiencing negative consequences, underestimate the amount of time spent on them, and use them as an escape from negative emotions, leading to a deterioration of interpersonal relationships. Effective preventive strategies include digital literacy education, parental interventions, promoting physical activity, and providing social support. Current research indicates that the most promising treatment approaches combine both therapeutic and pharmacological interventions. This chapter provides an in-depth exploration of the characteristics, impacts, symptoms, diagnosis, and strategies for preventing and treating Internet and smartphone addiction.