This study, funded by the Gates Foundation, reviews the currently available tools for and approaches to assessing the environmental impacts of livestock production systems. The demand for food from animal sources is expected to double by 2050, driven by population growth, urbanization and rising incomes. Demand in developing countries will account for the major part of the increase in both production and consumption of animal products. As a result, competition for land and water is likely to be fierce, with potentially profound outcomes for both the environment and food security. Furthermore, it will be imperative to limit agricultural expansion into vulnerable ecosystems to avoid irreversible changes in the resilience of agroecosystems. Thus, a large part of the demand must be met by the “sustainable intensification” of agriculture. There are many frameworks and methods for evaluating the environmental sustainability of farm systems. However, few of these initiatives are concerned solely with livestock systems, and these tend to focus on one or two areas rather than address all potential livestock-related environmental impacts.