While the field of measuring vertical inequality (VI, i.e. inequality among individuals in a society) is well developed and sophisticated, much less attention has been paid to measuring horizontal inequalities (HI), or inequalities among groups in a society. This paper proposes ways of measuring and monitoring horizontal inequalities. It is important to monitor HIs because they affect the well being of members of a group, and can be adverse to efficiency, growth and poverty reduction. Most important, they raise the risk of violent conflict. Yet for the most part, measurement and monitoring of HIs do not form a routine part of international or national data gathering and reporting.