This paper is the first to analyze eligibility and participation spells and estimate dynamic models of SSI participation by the aged. We first describe eligibility and participation spells and estimate  competing-risk models of the determinants of transitions. Next, we present evidence of extensive measurement error in the expected SSI benefit and the associated imputed eligibility status of sample members. We compare and contrast two approaches to ameliorating this error. A cross-section approach exploits self-reports of participants’ benefits, and a longitudinal approach makes inferences from time variation in the computed benefit. We find that the hazard model estimates vary little with regard to whether or which particular measurement error correction is employed. Finally, the longitudinal patterns of eligibility and participation suggest that take-up rates among the persistently eligible are nearly 80 percent.