Sustainability is a word that is frequently used and often without the understanding that it warrants. An understanding of sustainability is vital today as people need to adopt strategies to cope with issues caused by environmental degradation, social inequity and economic disparity. Education for sustainability has gained momentum after calls by international bodies to provide a vehicle for change to prioritise sustainability and integrate sustainable development at all levels of education. This thesis investigates what secondary school students understand about the complex concept of sustainability. It explores students’ conceptions of sustainability, their views about sustainability issues and the relationship between the students’ conceptions and their views about sustainability issues. This research used a mixed method approach to collect quantitative and qualitative data from questionnaires and follow up focus group interviews. Open and closed questions were used to investigate students’ conceptions of sustainability and these were compared to conceptions of sustainability that educators and scholars regard as important for citizens to hold for a sustainable future. Students’ views about sustainability issues were explored using modification of semantic differentials that were directly related to principles of sustainability that had been drawn from the literature. Data gathered were analysed using a thematic approach and simple statistical tests. Findings showed that students in this study had some understanding of sustainability. A minority of students held simple, one dimensional conceptions of sustainability but a significant number of students held conceptions that were more complex and multi-dimensional in nature. There were no students who held the expert conceptions of sustainability that scholars and educators regard as important, where environmental, economic, cultural, social and political conceptions are interrelated within an inter-generational setting. Findings also showed that students’ views of sustainability issues, determined from a series of differential statements, indicated that the majority of students agreed or strongly agreed with the sustainable view for most of the environmental, economic, cultural, social, political and inter-generational sustainability issues presented to them. Comparison of these findings indicated that a student who held a more complex conception of sustainability, and was able to identify a number of conceptions of sustainability, is more likely to make a sustainable choice on an issue if given the opportunity. These findings are encouraging but, because of the importance of sustainability, they suggest a need to further develop students’ understandings of this complex concept.