The purpose of this study was to investigate what Greek secondary school students (grades 8 and 11) believe about the greenhouse effect and climate change. A total of 626 students completed a closed-form questionnaire consisting of statements regarding the causes, impacts and solutions for this global environmental issue. The possible influence of three factors – i.e. educational level, gender and previous participation in Environmental Education extracurricular programs – on students‟ ideas was examined. The results suggest that eleventh graders were much better informed than eighth graders although some of the misconceptions reported in the literature (such as the cause-effect relationship between the greenhouse effect and ozone layer depletion) persist, irrespective of educational level. Students have fairly clear ideas of impacts while they seem to be somewhat confused about solutions and especially causes. Among the possible explanations of these trends, the logical sequence of physical consequences and students‟ difficulty in recognizing causes that presume specific scientific knowledge are emphasized. In addition, the role of information sources, especially television which emerged as the dominant source, is discussed. Finally participation in Environmental Education programs appears to be a critical factor since it has clearly influenced students‟ ideas in a positive way.