This treatment of the economics of social security examines the aims and operations of state social security systems, concentrating on the social "insurance" systems--pensions, unemployment, and sickness insurance. The authors analyze the rationale for a social insurance system, the appropriate level of state intervention, and the criteria used to judge a system's performance. The recent literature on the economic theory of insurance and on the labor supply, alternative methods of finance, and income reditribution is thoroughly discussed.