This document discusses how systems thinking can support sustainable development in monitoring and evaluation. It begins by defining sustainable development and the key aspects of sustainability. It then discusses theories of change and how they can be used to design for and evaluate sustained impact. Finally, it introduces systems thinking and how considering feedback loops, environmental conditions, adaptation, critical paths and other systems concepts can help create more sustainable programs and better evaluations by accounting for complex, dynamic systems. The overall message is that incorporating systems thinking into theories of change may help design more sustainable programs and conduct more useful evaluations.