The aim of this paper is to estimate the prevalence of Internet addiction among Greek University students. Results of a sample survey among 1875 Greek University students, ages 18-27, are presented. The questionnaire consisted of Young’s Diagnostic Test for Internet Addiction (YDTI A),14 as well as an inventory that included demographic factors and questions about the relations of students with parents, teachers and friends, their academic performance and their psychological characteristics. The consistency of YDTIA was tested with Cronbach’s alpha (0.71) with standardized items alpha (0.73). Results show that a) the prevalence rate is 13.12% b) Men are more addicted to the use of the Internet than women c) Academic performance, social relations and psychological characteristics of the students are impaired by the addiction to the use of the Internet d) The applications of the Internet which are most frequently used are: Chat, Sex Pages, Games, and e-mail e) Students studying more “applied” subjects, like Computer Science present a higher percentage of Internet addiction than students studying more “theoretical” subjects like Law or Business Administration.