Over three-fourths of the working-age population in the United States is insured for Disability Insurance (DI); this group is protected against a total loss of earned income typically associated with severe disability. However, little is known about the role the Supplemental Security Income (SSI) program plays in protecting against the financial consequences of severe disability for this population. We find that over one-third (36 percent) of the working-age population is covered by SSI in the event of a severe disability. Three important implications follow, which we discuss in sequence below: (1) SSI increases the overall coverage of the working-age population; (2) SSI enhances the bundle of cash benefits available to disabled individuals; and (3) interactions with other programs also enhance the safety net, most notably in the area of health insurance coverage. Ignoring these implications could lead to inaccurate inferences about disability program coverage, health insurance coverage, and the well-being of working-age individuals with disabilities. The first major finding is that SSI substantially increases overall cash benefit coverage. Thus SSI dramatically increases protection against the financial risk of disablement in the working-age population. While roughly 23 percent of the U.S. working-age population was not insured for DI in November 1996, SSI provides coverage for more than half of this seemingly "uncovered" population. An important innovation of our analysis is that we account for the possibility that many of those who appear ineligible for SSI based on current income could become eligible as a result of a disability shock that causes their earnings to drop. Thus the estimated proportion that is protected by SSI increases when the possibility of earnings loss because of disability is considered. Considering DI and SSI together, roughly 90 percent of the working-age population would be potentially covered for benefits in the event of a disability. Those who are covered by SSI--as opposed to those covered by DI alone-tend to be relatively young, less educated, and in relatively poor health. The remaining 10 percent or so are not covered by either DI or SSI. This group is economically vulnerable in some sense (they are poorer, older, and more likely to be women than those covered only by DI), but they are not as economically vulnerable in terms of income, resource holdings, and private health insurance coverage as those who are eligible for SSI. A disproportionate share of those who are not covered by either DI or SSI consists of married women. The second major finding is that SSI substantially enhances the bundle of available cash benefits. Roughly one-third of those covered by DI are initially covered by SSI as well. SSI enhances the bundle of available cash benefits through two mechanisms: (1) SSI provides cash payments during the 5-month DI waiting period, and (2) SSI supplements the DI benefit after the DI waiting period for people whose initial SSI payment is larger than the DI benefit. We find that the role of SSI cash payments is temporary for most of those who are initially covered by both SSI and DI: They would receive SSI during the DI waiting period, but would lose SSI eligibility afterwards because the higher DI benefit completely offsets the SSI benefit. However, a smaller group of DI beneficiaries with low DI benefit levels would continue to be covered by both SSI and DI after the DI waiting period because the relatively low DI benefit would not completely offset the SSI benefit. The third major finding is that interactions with other programs also substantially enhance the safety net. The most important interactions involve health insurance coverage. In the working-age population, Medicare is available to DI beneficiaries, but only after a 24-month waiting period. By contrast, SSI is an important pathway to Medicaid benefits for severely disabled adults with limited income and resources and has no waiting period. SSI can provide a pathway to health insurance coverage during the 24-month Medicare waiting period for some DI beneficiaries through providing access to Medicaid. Interactions with other programs, such as Temporary Assistance for Needy Families (TANF), Food Stamp, Unemployment Insurance (UI), workers' compensation (WC), and veterans' disability programs, modify the role of DI and SSI in protecting people against the adverse financial effects of disablement. The nature of the interactions with other programs differs depending on individual circumstances. Employment-related programs (including UI, WC, and veteran's disability programs) are particularly important for those who are covered by DI. By contrast, the means-tested programs (including TANF and Food Stamp) are more important for those who would be eligible for SSI. In conclusion, SSI plays a substantial role in protecting working-age people against the adverse financial consequences of disablement through three mechanisms: (1) providing coverage to many who are not DI insured; (2) providing additional cash benefits to many who are DI insured and also covered by SSI; and (3) enhancing the social safety net by interacting with other programs, most notably Medicaid. Through these mechanisms, the role of SSI is substantial enough that it cannot be safely ignored in econometric and policy research on DI.