This edited volume presents the social ontology of institutions. It questions what institutions are, what features and properties institutions have and what kinds of institution are present in the social world. The book answers these questions from both a speculative and an applied approach, it argues for a specific definition of institutions as a rule-based equilibria, as collective epistemic agent that is characterized by meaning, principles and power and as product of a We-mode and an imposition of a function. This book started from the interdisciplinary conference Playing by the Rules in Rijeka and contains contributions from Philosophy, Sociology and Economy. Institutions in Action is the first book to offer a comprehensive overview of the many different aspects and accounts about the social ontology of institutions. This much needed book presents researchers a very wide state of the art about the topic of institution by presenting the many differences that emerge in comparing the different positions.