The Social Security Administration (SSA) is responsible for administering two federal entitlement programs that provide income support to individuals with severe, long-term disabilities: Social Security Disability Insurance (SSDI) and Supplemental Security Income (SSI). SSDI is a work-related social insurance program that provides monthly cash benefits to nonelderly disabled workers and their eligible dependents, provided the workers accrued a sufficient number of earnings credits during their careers in jobs subject to Social Security taxes. In contrast, SSI is a need-based public assistance program that provides monthly cash payments to aged, blind, or disabled individuals (including blind or disabled children) who have limited assets and little or no Social Security or other income. Both programs use the same basic definition of disability to determine eligibility; however, by virtue of design, each program serves a somewhat different population. In 2017, SSDI and SSI combined paid an estimated $199 billion in federally administered benefits to 14.5 million qualified disabled individuals and 1.5 million non-disabled dependents of disabled workers. This report discusses the rules and processes used to determine eligibility for SSDI and SSI. It also explains how benefit amounts are computed, the types of non-cash benefits available to individuals who meet SSA's disability standards, and how each program is financed. For a quick overview of SSDI and SSI, see CRS In Focus IF10506, Social Security Disability Insurance (SSDI) , and CRS In Focus IF10482, Supplemental Security Income (SSI) . Old-Age, Survivors, and Disability Insurance (OASDI), commonly known as Social Security, is a federal social insurance program established under Title II of the Social Security Act that provides workers and their families with a measure of protection against the loss of income due to the worker's retirement, disability, or death. Workers obtain insurance protection by working for a sufficient number of years in jobs where their earnings are subject to Social Security taxes and therefore are creditable for program purposes. Social Security is financed largely on a pay-as-you-go basis, which means that payroll and self-employment tax contributions from current workers, their employers, and self-employed individuals are used to make monthly benefit payments to today's beneficiaries. In 2017, an estimated 173 million people (or about 94% of all workers) worked in paid employment or self-employment covered by Social Security, and the program paid monthly benefits to approximately 62 million beneficiaries ( Figure 1 ). The SSDI component of the program, which was enacted in 1956 and implemented in 1957, provides monthly benefits to statutorily disabled workers who are under Social Security's full retirement age (FRA) and to their eligible spouses, divorced spouses, minor children, student children, and disabled adult children. The Old-Age and Survivors Insurance (OASI) component of Social Security also provides benefits to eligible disabled dependents of retired workers and to eligible survivors of deceased beneficiaries and deceased insured workers. Although these individuals are not technically disability insurance beneficiaries, they are often included in the term SSDI , because they receive Social Security benefits due to a qualifying impairment. In December 2017, the SSDI component of Social Security paid benefits to 10.4 million individuals, including 8.7 million disabled workers and 1.7 million of their dependents. That same month, the OASI component paid benefits to 1.2 million OASI disability beneficiaries. To qualify for SSDI, disabled workers must (1) be insured in the event of disability, (2) be under Social Security's FRA, (3) have a qualifying impairment (see the " Definition of Disability " section of this report), and (4) have filed an application for benefits. Workers become insured for Social Security by acquiring a certain number of quarters of coverage (QCs) during their careers in paid employment or self-employment covered by Social Security. A worker's job is considered covered if the services performed in that job or net earnings derived by the individual result in wages or net earnings from self-employment income that are taxable and creditable for insured status and benefit computation purposes. In 2018, workers receive one QC for each $1,320 in covered earnings, up to the maximum of four QCs per year, regardless of when the money is earned. Thus, if a worker earns $5,280 in covered wages or net earnings from self-employment income during the first week of January 2018, then he or she would be credited with the maximum number of QCs for the calendar year. The amount of earnings needed for one QC is adjusted annually for average earnings growth in the national economy, as measured by SSA's Average Wage Index (AWI). Requiring individuals to have earned a certain number of QCs to qualify for cash benefits ensures that such individuals contribute a minimum amount to the insurance system via payroll and self-employment taxes on each QC's worth of covered earnings. To be insured in the event of disability, known as disability insured , covered workers must be both fully insured for Social Security and meet a recency-of-work requirement. To be fully insured for SSDI, covered workers must have at least one QC for each calendar year after they turned 21 years old and before the year they became disabled. The minimum number of QCs for fully insured status is six for the youngest workers (or 1.5 years of covered work); the minimum number of QCs needed for workers aged 62 or older is 40 (or 10 years of covered work). In effect, individuals must have worked in covered employment or self-employment for about a quarter of their adult lives to be fully insured. To meet the recency-of-work requirement , disabled workers generally need QCs during the 40-quarter period immediately before the onset of the disability. In other words, individuals must have worked in covered employment or self-employment for five of the 10 years before becoming disabled. However, workers under 31 years old may meet the recency-of-work requirement with fewer QCs based on their age. In 2017, SSDI provided disability insurance coverage to 154 million workers, with about 89% of covered workers aged 21-64 insured for SSDI. An insured worker must also be under Social Security's FRA to be entitled to SSDI, which for workers born from 1943 through 1954 is age 66. FRA is the age at which unreduced Social Security retired-worker benefits are first payable. Upon attaining FRA, disabled workers are automatically transitioned from disabled-worker benefits (or SSDI) to retired-worker benefits (or OASI); however, this change generally does not affect the amount of Social Security benefits paid to them or their dependents. Under current law, Social Security's FRA increases in two-month increments for workers born from 1955 through 1959 until reaching the age of 67 for workers born in 1960 or later. SSDI is not available to workers who have already attained FRA. In addition to the disabled worker's own benefit, SSDI provides benefits to certain family members of the worker. Social Security pays benefits to family members because workers with one or more dependents are presumed to have greater financial need when they retire, become disabled, or die than similarly situated workers who are single. The OASI component also provides benefits to eligible disabled dependents of retired workers and to eligible survivors of deceased insured workers. The term deceased insured workers includes deceased individuals who received Social Security retired or disabled-worker benefits and non-beneficiary workers who were insured for Social Security at the time of their deaths. Validly married spouses of disabled workers qualify for benefits if they (1) are aged 62 or older or are any age and have an eligible child in their care who is under the age of 16 or disabled, (2) are not entitled to a retired or disabled-worker benefit equal to or larger than the spousal benefit, and (3) have filed an application for benefits. Spouses also must have been married to the worker for at least one continuous year immediately before the day on which the claimant's application is filed. This provision is known as a duration-of-marriage requirement . Divorced spouses of disabled workers may qualify if they (1) are aged 62 or older, (2) are unmarried unless the remarriage occurred after attainment of age 60 or age 50 and the claimant was entitled to disabled widow(er)'s benefits, (3) are not entitled to a retired or disabled-worker benefit equal to or larger than the spousal benefit, and (4) have filed an application for benefits. Divorced spouses must have been married to the worker for at least 10 years immediately before the date the divorce became final. Eligible minor children of disabled workers qualify for benefits if they (1) are unmarried, (2) are under the age of 18, and (3) have filed an application for benefits. An eligible child is the natural (i.e., biological) child, adopted child, stepchild, equitably adopted child, grandchild, or step-grandchild of the insured worker on whose earnings record the claim is based. For certain child claims, such as those involving stepchildren, an explicit dependency requirement must be met, which generally involves the insured worker providing evidence that the child is living with the worker or is receiving at least one-half of his or her support from the worker. Otherwise, the child is presumed to be dependent on the insured worker for his or her support. For more information on the rules governing a child's status for purposes of SSDI entitlement, see "GN 00306.001 Determining Status as Child," in SSA's policy manual, the Program Operations Manual System (POMS). Eligible student children of disabled workers qualify for benefits if they (1) are unmarried, (2) are aged 18-19, (3) are a full-time student at an elementary or secondary school, and (4) have filed an application for benefits.