This paper develops a method to estimate workers’ outside employment opportunities. We outline a matching model with two-sided heterogeneity, from which we derive a sufficient statistic, the “outside options index” (OOI), for the effect of outside options on earnings, holding worker productivity constant. The OOI uses the cross-sectional concentration of similar workers across job types to quantify workers’ outside options as a function of workers’ commuting costs, preferences, and skills. Using German micro-data, we find that differences in options explain 20% of the gender earnings gap, and that gender gaps in options are mostly due to differences in the implicit costs of commuting and moving.