This book inquires into the Capability Approach, a value theory of freedom, which crystalizes the interests of Marx, Welfare Economics, Social Choice, and Ethics. The capability approach has attracted many people as a promising interdisciplinary approach to human well-being and social worlds, finely overarching ethical and economic concerns. It has well challenged essential characteristics of welfare economics, which focuses on the criterion of efficiency with the concept of utility, by explicitly incorporating normative criteria such as agency, well-being and real freedom into positive analysis. However, it has a bit operational and methodological difficulties such that how to estimate an individual capability set which includes potential multi-dimensional functioning vectors. This book reminds the reader of what traditional economics has left behind, by examining historical backgrounds, scrutinizing philosophical foundations and providing an operational formulation of the capability approach: indispensable for understanding what the capability approach is about and what it can achieve.