This paper presents a framework for understanding the role that systems theory might play in education for sustainability (EfS). It offers a sketch and critique of Land and Meyer's notion of a "threshold concept", to argue that seeing systems as a threshold concept for sustainability is useful for understanding the processes of learning for sustainability. With an understanding of systems approaches as a key part of the practice of sustainability, educators do well to focus on ways to facilitate learners' internalisation of systems thinking through EfS. This is particularly relevant given that sustainable development requires learners to go out into the world and apply sustainability theory to create change. The capacity to do this, it is argued, depends on learners' understanding and internalisation of the core concepts of sustainability, such as systems.