According to recent epidemiological data, 18.9% of the world population are internet users (www. internetworldstats.com). Τhis article attempts to determine the phenomenon of internet addiction disorder and to analyse the concepts of addiction and compulsiveness. The symptoms of internet addiction will be presented; the prevalence and demographic characteristics of internet addicts will be shown. Diagnostic tools developed for internet addiction, most susceptible populations, type of their personalities and integration of internet addiction disorder into the DSM-IV will be investigated in the present article. During the last decade, mental health specialists' research studies on internet addiction have not reached a consensus concerning the name, diagnostic criteria, diagnostic tools or introduction into the DSM-IV system. Nowadays, new technologies have penetrated into everyday life, while their influence is considerably growing, as the number of internet users is increasing. Therefore, it seems important for clinicians to include in their diagnostic interviews with patients questions investigating their relationship with the new technologies, as well as to recognize their impact in their patients' way of living.