Livestock industries strive to improve the health of their animals and, in the future, they are going to be required to do this with a continued reduction in antimicrobial use. Nutraceuticals represent a group of compounds that may help fill that void because they exert some health benefits when supplemented to livestock. This review is focused on the mechanisms of action, specifically related to the immune responses and health of ruminants. The nutraceutical classes discussed include probiotics, prebiotics, phytonutrients (essential oils and spices), and polyunsaturated fatty acids. Copyright © 2019 Elsevier Inc. All rights reserved.