Our understanding of the immune system emerged from the study of disease processes and the communication networks used by various cell types to respond to pathogens. As with many aspects of physiology, this initial view was colored by the techniques available at the time. With technical advances in the beginning in the 1990’s, research in sepsis and obesity began to identify critical interactions between the immune system and metabolism. Our current understanding of these interactions is informed by two active but largely distinct research communities. Many in the field of immunology are utilizing cellular metabolism tools to understand mitochondrial function and fuel use in response to activation of innate and adaptive immune cells, especially as these relate to cancer. From another vantage point, many metabolic physiologists are now seeking to understand the importance of tissue-resident immune cells and immune signaling molecules in metabolic homeostasis and pathologies. Beyond human health implications of recent findings, a number of immunometabolism insights have informed our understanding of livestock health. In inflammatory events, phagocytic cells are activated, and the dramatic increase in oxidative metabolism is driven primarily by glucose use. Metabolism of healthy animals is also influenced by secretions from immune cells. Studies in mice indicate that appropriate host/microbe interactions (balancing protection and tolerance) are mediated by a network of immune cell types in the gut, which is critical to both absorptive and barrier functions of the gut. Adipose tissue immune cells regulate lipolytic rate, insulin sensitivity, and perhaps whole-body inflammatory tone. Local immune cell impacts on metabolism of other organs, including the liver and pancreas, are also emerging. Immunity and metabolism are tightly interwoven, and the evolving understanding of these links may enable nutritional or pharmacological strategies to enhance resilience to disease and alter nutrient partitioning in livestock.