This monograph explores the foundations of welfare economics - in particular the assessment of personal well-being and advantage. Arguments are presented against the usual concentration on ``utility'' (as in traditional welfare economics) and on ``opulence'' (as in ``real income'' estimations and other normative measurements). An alternative framework is developed for the analysis of personal well-being based on assessing ``functionings'' and the ``capability to function'' - i.e. what a person can do or can be. The functionings considered vary from such basic biological performances as ``being well-nourished'' to such social achievements as ``being able to take part in the life of the community''. This exercise of comparing achievements and capabilities provides a structure for inter-personal comparisons of well-being. The book's critical appraisal of traditional methods combined with a newly developed framework throws new light on comparisons of well-being, and aids assessment of some international contrasts.