Since the banning of antibiotic use in animal feeds in 2006 there has been much interest in supplementary feed additives to support animal production, efficiency and health. Furthermore, these supplements may offer the opportunity for reductions in enteric methane production. This chapter reviews a number of these additives and their benefits for sheep in particular, including prebiotics, probiotics, plant extracts, macroalgae, alkaloids, flavonoids and chemical additives. The different types of supplements within these categories are also outlined, such as yeast cultures and essential oils and their potential mode of action.