In order to be sustainable, sheep rearing systems must be sufficiently flexible to respond to variable conditions and feed availability. This chapter argues that sustainable sheep management works with the resources available at a given place or time and learns from the sheep themselves. The chapter further describes how sheep cope with a variable supply of nutrients, and examines in detail (including via a case study) the importance of understanding the potential variety and usefulness of forages. The chapter also addresses how animals learn about forages, and concludes that sheep production can be made sustainable by providing diverse forages suited to the local environment and by managing the experiences of animals so they learn to combine appropriate mixtures of plants as sources of nutrition.