Multi-Robot Systems (MRS) are, nowadays, an important research area within Robotics and Artificial Intelligence and a growing number of systems have been recently presented in the literature. In this paper we present an analysis of the most relevant works on MRS by specifically examining their cooperative aspects. In particular, we propose a new taxonomy for a fine and precise analysis of the recent works on MRS and we describe some approaches which we consider representative in our taxonomy. We finally discuss the outcome of our analysis and try to highlight future trends of the research on MRS.