Diversified farming systems are proposed as a major mechanism to address the many sustainability issues of today's agriculture. Multi-species livestock farming, i.e. keeping two or more animal species simultaneously on the same farm, is an option that has received little attention to date. Moreover, most studies of multi-species livestock farming are limited, usually focusing on selected dimensions of farm sustainability and addressing lower organizational levels (i.e. within the farm) and rather limited time horizons (e.g. a few weeks in a grazing season). Thus, a comprehensive assessment of multi-species livestock farming in terms of farm sustainability is lacking. In this context, we outline and discuss potential benefits and limitations of multi-species livestock farming for livestock farm sustainability from existing literature and list issues on multi-species livestock farming requiring further research. We show that multi-species livestock farming has the potential to improve the three dimensions of sustainability reviewed - economic viability for farmers, environmental soundness and social acceptability by being respectful of animals and humans - as long as locally relevant farming practices are implemented, especially an appropriate stocking rate during grazing. If relevant practices are not observed, multi-species livestock farming may produce undesirable effects, such as competition for resource acquisition during grazing, parasitic cross-infection and more intense work peaks. Therefore, we identify four focal research areas for multi-species livestock farming. First, characterizing the management of multi-species livestock farms. To do this, we suggest considering the integration of production enterprises (e.g. cattle and sheep enterprises) within the farm from three perspectives: farming practices (e.g. grazing management), work organization and sales. Second, exploring the complementarity of livestock species on multi-species livestock farms. This is especially true for species combinations that have been largely ignored (e.g. ruminants and monogastrics), even though they may have potential due to complementary diet compositions and resource-acquisition strategies. Third, assessing the sustainability of multi-species livestock farm scenarios (current or alternative) according to the management practices and production conditions, which requires adapting existing methods/models or developing new ones. Fourth, characterizing conditions for success and obstacles for multi-species livestock farming along the value chain from production to consumption, considering stakeholders' objectives, work habits and constraints. Increasing understanding should help prioritize actions and organize them to scale up multi-species livestock farming.