Problem Statement: Excessive internet use, particularly problematic and negative consequences of internet use, is rapidly increasing among children and adolescents throughout the World and in Turkey. While the internet provides potential advantages for users in terms of the academic support, sharing ideas & feelings, and freedom of expression, it also has some adverse effects such as wasting time, exhibiting antisocial behaviors, and causing relationship problems. Excessive use could also be manifested as absenteeism, academic failure and problems in interpersonal relationships with other students. In this context, determining the motivations behind the excessive use of the internet could be beneficial in an educational setting. Purpose of the Study: The purpose of this study is to determine the aims, causes and consequences of excessive internet use among adolescents and examine the aims of excessive internet use in terms of gender and grade level. Method: In this study, a quantitative method was used as the research design. As the research aims require in-depth opinions, data triangulation was also used. The study group consisted of 91 high school students who use the internet excessively. The study group were selected by screening 1,587 students living in the Istanbul province during the 2012-2013 Academic year who were internet users for "41 hours or more a week." Data was collected by the Internet Use Purposes Scale (IPUS) and a semi structured questionnaire. Findings: The purposes of excessive internet use among these adolescents were socialization, learning and general culture, meeting psychological needs, entertainment, and education. There are a significant differences between the opinions of the adolescents in the entertainment and learning & general culture sub-dimensions in terms of gender and in the psychological needs sub-dimension in terms of grade level. Causes of excessive internet use are grouped under the main categories of learning and development needs, socialization need, psychological reasons and seeking entertainment. Consequences of excessive internet use fall under the main categories of social, physical health, psychological and mental problems. Conclusions and Recommendations: In view of the purposes, causes and consequences of excessive internet use in adolescents, the need to socialize and learn are emphasized in terms of purposes and causes. Boys use internet excessively more often than girls for the aims of entertainment and information gathering. Psychological needs are prevalent in 9 and 12 grades; excessive use causes multifaceted adverse effects on adolescents, including social, physical, academic, psychological and mental problems. In this context, meaning of "knowledge" and "socialization" for the adolescents should be researched by the academia. Programmes should be designed to meet the students' learning and psycho-social developmental needs at schools and in real life. Artistic, sports and cultural activities must be increased in real life and school settings.