Algorithmic Institutionalism is the first book to conceive algorithms as institutions in contemporary societies, focusing on different dimensions of how they structure decision-making and enact power relations. In many situations in contemporary societies, algorithms structure social interactions, resulting in patterns of action and human behavior in collective contexts. Almeida, Filgueiras, and Mendonca discuss how algorithms are gradually occupying an institutional space in societies, deciding on different aspects of social life and shaping collective and individual human behaviors. As institutions, algorithms work as decision systems that define what is allowed, hindered, facilitated, or made impossible as well as positions within society's organizational structures. Algorithmic institutionalism uses the perspective of institutional theories to explain the functioning of these decision systems and how they establish patterns and norms that affect human behavior and lead to deep changes in contemporary society. The book points to the challenges of political orders that are gradually institutionalized with algorithms, comprising new dynamics of interaction between humans and machines. These disruptive dynamics of interaction between humans and machines create new challenges related to the democratization of algorithms and the impasses that emerge with technological advancement through digital technologies. Providing an analytical framework for an adequate comprehension of the social and political implications of algorithmic systems, Algorithmic institutionalism applies this framework to make sense of recommendation systems, the platformization of governments, and the deployment of algorithms in security. It then addresses the challenge of developing approaches to democratize the new political order influenced by the global expansion of algorithmic decision-making, pointing to key democratic values that are relevant once we consider the construction of legitimate decisions in contemporary societies.