We conducted a direct mail field experiment with 4,016,461 individuals to test several key hypotheses about why take-up of Supplemental Security Income among individuals age 65 and above is so low. Communicating likely eligibility in a basic letter generated substantial increases in take-up in relative terms. Adding behaviorally informed statements increased the effectiveness of these communications. Yet, the application rate in our study sample during the full 24-month follow-up period remained no greater than 7 percent. Our results reveal a modest trade-off between increasing applications and the conditional likelihood of award, as well as the award amount. (JEL C93, D12, D83, H55, I38, J14)