This study attempts to explain the way Sen defines economic growth and social development. His approach is derived from an analysis of individual behavior that makes a distinction between Resources, « Functionings » and « Capabilities ». These concepts have evidently implications concerning the measure of social welfare. The paper summarizes also the main elements of the theory of inequality and poverty measurement, topics to which Sen made basic contributions. The study ends by a description of Sen's analysis of sexual and racial discrimination, his thesis on the central role that should be given to mortality as a measure of economic success or failure and his work on malnutrition and famines.