The Handbook of Social Choice and Welfare presents, in two volumes, essays on past and on-going work in social choice theory and welfare economics. The first volume consists of four parts. In Part 1 (Arrovian Impossibility Theorems), various aspects of Arrovian general impossibility theorems, illustrated by the simple majority cycle first identified by Condorcet, are expounded and evaluated. It also provides a critical survey of the work on different escape routes from impossibility results of this kind. In Part 2 (Voting Schemes and Mechanisms), the operation and performance of voting schemes and cost-sharing mechanisms are examined axiomatically, and some aspects of the modern theory of incentives and mechanism design are expounded and surveyed. In Part 3 (structure of social choice rules), the positional rules of collective decision-making (the origin of which can be traced back to a seminal proposal by Borda), the game-theoretic aspects of voting in committees, and the implications of making use of interpersonal comparisons of welfare (with or without cardinal measurability) are expounded, and the status of utilitarianism as a theory of justice is critically examined. It also provides an analytical survey of the foundations of measurement of inequality and poverty. In order to place these broad issues (as well as further issues to be discussed in the second volume of the Handbook) in perspective, Kotaro Suzumura has written an extensive introduction, discussing the historical background of social choice theory, the vistas opened by Arrow's Social Choice and Individual Values, the famous "socialist planning" controversy, and the theoretical and practical significance of social choice theory. The primary purpose of this Handbook is to provide an accessible introduction to the current state of the art in social choice theory and welfare economics. The expounded theory has a strong and constructive message for pursuing human well-being and facilitating collective decision-making.