Loneliness is a disturbing and universal human experience, which, to a greater or lower extent, all people experience throughout life (Peplau and Perlman, 1982). Many authors point out that loneliness is especially expressed in adolescence due to the great developmental changes that mark this period. In recent decades, there is a growing scientific interest in the study of the psychological effects of the use of the Internet because of its increasing use and the development of problematic Internet use. The main aim of this study was to investigate differences in loneliness and problematic Internet use among high school students and university students, differences in problematic Internet use considering the level of loneliness, and the relationship between loneliness, problematic Internet use, and some aspects of using the Internet. The study was conducted on a sample of 404 participants, high school students and university students. In order to provide an answer to the identified problems two scales were applied: a short version of the UCLA loneliness scale and the scale of generalized problematic use of the Internet 2. The results of this study showed that there was no statistically significant age difference in loneliness and problematic Internet use, in other words there was no difference between high school students and university students. Furthermore, in accordance with previous studies it was found that lonely individuals have more pronounced problematic internet use. Additionally, there was a significant positive correlation between loneliness and problematic Internet use, while no significant correlations between loneliness and some aspects of using the Internet were found. Furthermore, there was a positive correlation between problematic Internet use and the average time that individuals spend online, that is, individuals with more pronounced problematic Internet use spend more time on the Internet daily.