Grazed pasture is the single most important forage feed for ruminants due to its low unit cost and widespread global availability. However, there is a growing use of arable crops in ruminant rations, which may exceed that used by humans by 2050. This chapter describes the ways in which grazed pasture provides the energy, proteins, minerals and vitamins and other nutritional factors required by ruminants. The chapter shows that the use of pasture and its by-products underpins the possibility of sustainably delivering future ruminant livestock production systems and ensuring their future role in food security.