Environmental education has been included in the school curricula of many countries in the last 15 years. Implementation has been not as successful as expected due to various constraints reported in several research projects. Its achievement remains uncertain and more research is required in all areas of education. In order to propose a curriculum change, research is needed on how and what students really understand and know concerning sustainable development. An exploratory study was undertaken to study levels of comprehension of certain themes of sustainable development in adolescents. Interviews were conducted with 40 Spanish and Mexican high school students in aspects relating to waste and recycling, several forms of pollution and contamination, ideas on scarcity of natural resources, and population increase. Data illustrate that students are aware of environmental issues, but lack knowledge of how ecological, social and economic aspects are related in sustainable development, showing that information provided in school has not been enough for students to consider these aspects and use them for the fostering of citizenship participation. Knowing the concepts that adolescents use to explain sustainable development, as well as knowledge and information gaps, can allow us to improve environmental education.