 The human appetite for the food products from livestock, meat and milk, is seemingly insatiable. Worldwide, four-fifths of all agricultural land is dedicated to raising livestock, and with the growth in population and wealth in developing countries, that appetite is only getting bigger. By 2050, the demand for meat and milk is expected to increase by 80%. However, there is growing concern about how to accommodate this increase in demand with a low environmental footprint, and without eroding the economic, social, and cultural benefits that livestock provide. And herein lie many challenges. One is that meat and milk supply only 15% of the total energy and 30% of the protein in the average global human diet, despite taking up 80% of all agricultural land for their production. On top of that, livestock already emit large quantities of greenhouse gases and use 25-32% of global fresh water. A third batch of problems is created by climate change itself. Heat stress and water scarcity affect animals and the production of their feed, and alter the prevalence and intensity of disease. Over the past 10 years, a great deal has been learned about the environmental impacts of livestock. We now have an idea of what might constitute a more sustainable system, but we don't quite know how to get there. What's urgently needed is research to develop efficient and culturally equitable pathways for transitioning to more sustainable livestock systems. Let's look at the emissions issue in more detail. The 17 billion food-producing animals across the globe collectively emit between 8 and 18% of greenhouse gases. Most of this is methane generated in the digestive tracts of ruminants, animals such as cattle, sheep and goats. Another portion is emitted as carbon dioxide as a result of changes made to soil and plants to accommodate livestock. So how might these emissions be managed? Consider global livestock farming as a factory. Some livestock systems utilize resources much more efficiently than others. Specifically, pork and chicken show better environmental performance per unit of product than dairy or beef cattle. To output quality products with low emissions of harmful gases, you need to manage the input. For farmers, that means providing animals, especially cattle, with feed containing additives that curb the formation of methane, or using high-quality feed that is packed with energy and is easier to digest. Another major input that can be controlled is the land used for grazing. Because plants and soil are enormous sinks for carbon dioxide, over-sowing grasslands with crops like legumes and avoiding massive changes in land cover promotes the storage of carbon dioxide, reducing the effects on the environment. Changing consumer behavior is another goal. Reducing the consumption of animal products would free up space for forests and grasslands that capture carbon dioxide from the atmosphere, as well as promote a healthier diet. However, studies to date have been unable to fully capture the indirect effects that shifts in consumption habits could have on land use, agricultural productivity, and even household budgets. The design and adoption of practical, sustainable solutions remain a considerable challenge, a global one. Taking on this challenge requires incentives and other policies for affecting changes in how livestock are raised and how their products are consumed. All to ensure that livestock contribute to the sustainable and nutritious diets of different age groups of the human population. Most importantly, these efforts must be regionally and culturally sensitive. Now is the time to move from knowledge to action, from assessing the world's needs, to figuring out how to satisfy them.