How can groups - e.g., committees, expert panels, collegial courts, legislatures, electorates - make coherent collective judgments on interconnected issues based on their members' individual judgments? The theory of judgment aggregation provides a general framework for studying this question, extending social choice theory in the tradition of Nicolas de Condorcet and Kenneth Arrow. This book introduces the theory, explains its central impossibility results, and shows how they can be avoided, especially by means of a holistic approach in which webs of connected propositions, rather than individual propositions, are the unit of aggregation. The book further investigates the role of deliberation, information-based revision of judgments, strategic manipulation by voters and agenda setters, truth-tracking, and the aggregation of probabilities, estimates of variables, and other non-binary judgments. The book gives a unified perspective on the field and highlights promising areas for further research. This title is also available as Open Access on Cambridge Core.