BACKGROUND: Informal employment (IE) is one of the least studied employment conditions in public health research, mainly due to the difficulty of its conceptualization and its measurement, producing a lack of a unique concept and a common method of measurement. OBJECTIVE: The aim of this review is to identify literature on IE in order to improve its definition and methods of measurement, with special attention given to high-income countries, to be able to study the possible impact on health inequalities within and between countries. METHODS: A scoping review of definitions and methods of measurement of IE was conducted reviewing relevant databases and grey literature and analyzing selected articles. RESULTS: We found a wide spectrum of terms for describing IE as well as definitions and methods of measurement. We provide a definition of IE to be used in health inequalities research in high-income countries. Direct methods such as surveys can capture more information about workers and firms in order to estimate IE. CONCLUSIONS: These results can be used in further investigations about the impacts of this IE on health inequalities. Public health research must improve monitoring and analysis of IE in order to know the impacts of this employment condition on health inequalities.