Social Choice theory is the study of collective decision processes, their procedures and their consequences on welfare. It studies questions such as how to aggregate individual preferences into social preferences; what the properties of different voting systems are; what the resource allocation mechanisms are that take into account the incentives of the participants. The aims of Social Choice are twofold: first, to study particular models that are important for its application to real life (such as the study of the outcomes obtained by run-off voting systems); and second, to take a normative point of view in designing ideal systems (such as the design of decentralized allocation mechanism for the allocation of public goods or institutional design). The tools used range from axioms to game theory.