This paper examines the redefault rate of mortgages that were selected for modification during 2008-2011, compared with that of similarly situated self-cured mortgages during the same period. We find that while the performance of both modified and self-cured loans improved dramatically over this period, the decline in the redefault rate for modified loans was substantially larger, and we attribute this difference to a few key factors. First, the repayment terms provided by modifications became increasingly generous, including the more frequent offering of principal reduction, resulting in greater financial relief to borrowers. Second, the later modifications also benefited from improving economic conditions -modification became more effective as unemployment rates declined and home prices recovered. Third, we find that the difference in redefault rate improvement between modified loans and self-cured loans is not fully explained by observable risk and economic variables. We attribute this residual difference to the servicers' learning process -so-called learning by doing. Early in the mortgage crisis, many servicers had limited experience selecting the best borrowers for modification. As modification activity increased, lenders became more adept at screening borrowers for modification eligibility and in selecting appropriate modification terms.