Sustainability is one of the most important challenges for present and future societies, which needs to be urgently and adequately addressed from Education. This study qualitatively analyses the curricula of primary and secondary education in Spain and Portugal, with the objective of assessing how sustainability is addressed across compulsory education in both countries. The results show a limited presence of sustainability in the curricula of both countries, the coexistence of different and sometimes inconsistent conceptions of sustainability, deficiencies in the inclusion of the social and economic dimensions, and a questionable technological optimism as the solution to sustainability problems. Such findings provide with a basis to improve curricula, also in other countries. Future curricular designs should seek a more ambitious approach to sustainability based on the equal importance of its three dimensions, an effective cross-curricular character, critical thinking, and an ecocentric ethics.