A healthy diet starting in the early years of life is essential for the development of physical and mental wellbeing. As has been shown by research conducted internationally, students’ knowledge about nutrition is correlated with their eating habits. The present study aims to investigate Greek adolescent students’ knowledge of nutrition concepts. Specifically, because of the lack of similar research about Greek adolescents, we sought a first mapping of trends so that in the future we could design appropriately oriented teaching interventions. The survey was conducted through a questionnaire, and involved 269 students aged 15–16 years from schools across Athens. The results showed that while the students’ knowledge of some aspects of nutrition seems to be quite good, knowledge of other aspects is extremely poor. Furthermore, even when relatively proficient about nutrition, some participants ignored that knowledge in their daily lives. According to the students, the main sources of information on nutrition issues are their parents and the internet, followed by school. These findings indicate a need to focus on designing coherent educational proposals on nutrition concepts, which are not only important for their own sake, but also for the sake of long-term health.