Jean Charles Léonard de Sismondi, also known as Jean Charles Leonard Simonde de Sismondi,, whose real surname was Simonde, was a Swiss historian and political economist, who is best known for his works on French and Italian history, and his economic ideas. His Nouveaux principes d'économie politique, ou de la richesse dans ses rapports avec la population (1819) represents the first liberal critique of laissez-faire economics. He was one of the pioneering advocates of unemployment insurance, sickness benefits, a progressive tax, regulation of working hours, and a pension scheme. He was also the first to coin the term proletariat to refer to the working class created under capitalism, and his discussion of mieux value anticipates the concept of surplus value. According to Gareth Stedman Jones, "much of what Sismondi wrote became part of the standard repertoire of socialist criticism of modern industry," earning him critical commentary in the Communist Manifesto.

The proletariat is the social class of wage-earners, those members of a society whose only possession of significant economic value is their labour power. A member of such a class is a proletarian or a proletaire. Marxist philosophy regards the proletariat under conditions of capitalism as an exploited class⁠—  forced to accept meager wages in return for operating the means of production, which belong to the class of business owners, the bourgeoisie.