Different conceptions of inequality of opportunity have been developed in political philosophy. We focus on two competing conceptions: fair and luck egalitarian inequality of opportunity. We develop an empirical framework to measure and compare these conceptions. Both fair and luck egalitarian inequality of opportunity consider it unjust if ascribed characteristics such as social origin, gender, race, and ethnicity affect life outcomes. However, they differ in what they regard as just: fair inequality of opportunity considers it just if abilities and individual choices affect life outcomes, while luck egalitarian inequality of opportunity considers it just only if individual choices affect life outcomes. We measure fair and luck egalitarian inequality of opportunity in education, occupation, income, and wealth using survey data from the National Longitudinal Survey of Youth (NLSY). Our results show that conclusions about inequality of opportunity depend on the conception of inequality of opportunity employed. From the perspective of fair inequality of opportunity, the United States is a rather just society, while the nation’s degree of luck egalitarian inequality of opportunity is high. The difference between fair and luck egalitarian inequality of opportunity is largest for education, followed by occupation, and finally income and wealth. There is no evidence that changes across cohorts differed between fair and luck egalitarian inequality of opportunity.