This document discusses international and national definitions of the informal sector. It summarizes that the 15th International Conference of Labour Statisticians adopted an international statistical definition of the informal sector in 1993 based on characteristics of unincorporated enterprises. However, national definitions vary in terms of data sources, coverage, criteria used to define the informal sector, and treatment of specific groups. These differences affect the international comparability of informal sector statistics. Harmonizing national definitions with the international framework would enhance comparability.