The possibilities offered by the use of the Internet increasingly intensify the problem of Internet addiction, which has become more prevalent in the last decade, marked by the growing availability of mobile devices and new media and their exacerbation of the problem. Research on Internet addiction, initiated by Kimberly Young at the end of the twentieth century, usually appears in the literature in the context of young people who have been found to be most vulnerable. The phenomenon is known as Adolescent Internet Addiction. Compulsive use of the Internet is a complex phenomenon, its effects being visible in almost all aspects of a young person’s social life. It is manifested in a variety of pathological behaviors and emotional states grouped into several major psycho-physical and social effects that may appear simultaneously, e.g. anger, depression, loneliness or anxiety associated with the lack of access to the network, the weakening of social ties, withdrawal from real life, lack of educational achievement, chronic fatigue or deteriorating health. The authors of this study aim to assess the level of Internet addiction among adolescents in Poland and indicate its main behavioral manifestations, in the students surveyed, which influence their pathological use of the Internet. Our study involved a total of 505 students from three high schools located in Rzeszow (N = 505) and was carried out by questionnaires, including, among others, The Problematic Use of the Internet (PUI) which is the Polish adaptation of Kimberly Young’s Internet Addiction Test (IAT) (Cronbach’s α = 0.89). Statistical analysis of responses from the PUI test allowed us to determine (1) the level of Internet addiction among these adolescents, whereas the univariate (ANOVA) analysis enabled us (2) to verify the hypothesis of the existence of differences in the level of Internet addiction among the investigated groups as far as gender, place of residence or grade are concerned. Generally, the results obtained in our research indicate that the level of Internet addiction among the adolescents investigated is not very high, although two thirds of our respondents showed an above average level of addiction, and every ninth respondent (approximately 11%) was highly addicted to the Internet, men being more often addicted (15.6%) than women (8.3%).