This chapter examines the impact of improving feed efficiency on the environmental impact of livestock production. It starts by discussing the relation between greenhouse gases and dairy production, highlighting how important it is to the dairy sector to find ways of decreasing greenhouse gas output. The chapter then moves on to discuss the origins of methane and reactive nitrogen excretions in ruminants. A section on improving feed conversion efficiency is also included, which is then followed by a review of the nutritional practices that can be used to enhance feed conversion efficiency and decrease methane excretion. The chapter also examines the nutritional practices that can be used to increase milk protein efficiency and nitrous oxide excretion as well. Discussions on genetics and feed conversion efficiency and postabsorptive metabolism and feed conversion efficiency are also provided.