The present research studies the knowledge of 221 Ioannina’s city, located in the northwestern region of Greece, high school students on 10 environmental issues. Schoolers’ responses were collected through a “closed response” questionnaire distributed at three of the city’s Public High Schools. The overall picture appeared to be moderately satisfactory, as high schoolers possess a good knowledge on several environmental issues, while misconceptions on other issues, such as sustainable development, predominate. Taking into account the outcome of the present study we consider that improvements are needed in the existing Environmental Education Program in order to ameliorate their knowledge.