SSDI pays disability benefits to individuals who are insured due to contributions to the Social Security trust fund through Social Security tax on their earnings. SSDI also makes payments to certain people with disabilities who are dependents of insured individuals.
SSI provides for payments to persons, including children under the age of 18, who have disabilities and limited income and resources.
The SSA, which makes decisions about eligibility for disability benefits, defines disability as the inability to engage in substantial gainful activity because of any medically determinable physical or mental impairment which has lasted or is expected to last at least 12 months or is expected to result in death. Disability Determination Services operations are fully funded by the SSA.