Multi-robot systems have evolved significantly in the last forty years. Many of their research and development challenges extend those of single robot systems, but many others are intrinsic to their distinctive nature. In this chapter we cover, using a systems-based view and, in most cases, a probabilistic framework, four of the major research fields in multi-robot systems: cooperative perception, cooperative localisation, formation control, and task allocation, cooperative planning and plan execution. Some of the major research directions in those fields are tackled, including formal models and illustrative cases available in the literature. The chapter ends with a list of future directions for research and applications involving multi-robot systems.