This chapter suggests that devising an ironclad definition of sustainable food systems is neither possible nor desirable. The movement for sustainable food systems has become one of the more exciting and pivotal movements of our time, tying together issues of urgent importance and animating growing numbers of sometimes disparate people and social groups. The qualified and still incomplete experience in Pennsylvania measuring and monitoring sustainability-relevant trends in the food system illustrates some of the broad challenges of reductionism, agenda-setting and politics that Bell and Morse identify in efforts to develop sustainability indicators. Sustainable agriculture has most readily focused on the environmental or ecological soundness of the production system or agro-food commodity chain. Arising from recognition of the intractable food and income security challenges facing the rural poor in many developing countries, sustainable livelihoods are to be analyzed and facilitated at the household and community levels.