The notion of equality of opportunity has motivated different empirical approaches. However, not all empirical approaches to measure equality of opportunity have been motivated by the same conception of equality of opportunity. This book chapter distinguishes between two conceptions of equality of opportunity, liberal and radical equality of opportunity, and relates them to empirical approaches that aim at measuring equality of opportunity: research on intergenerational mobility, measures of sibling similarities, surname mobility, and the equality of opportunity measurement advanced by John Roemer. The chapter concludes with some directions for further research, suggesting that (1) empirical research should measure and compare both liberal and radical equality of opportunity and (2) more empirical progress will come about by combining the different approaches to measure equality of opportunity.