This chapter addresses the contributions of Karl Marx, Michal Kalecki and John Galbraith to these power issues. Firstly, we begin with Karl Marx who pointed out the centrality of class analysis to the study of power in capitalism. Then, we consider the analysis of Michal Kalecki on the degree of monopoly. Proceeding from a Marxist tradition, his analysis highlights important contemporary issues related to power in market structures and their outcomes in terms of income distribution. Finally, we present the main ideas of John Galbraith on the modern institutions. He deeply analysed the institutional set up of large corporations and highlighted their power in the markets and their relations with the nation-state.