Difference between Social Security Disability Insurance (SSDI or Title II) and Supplemental Security Income (SSI or Title XVI)

The Social Security Administration is responsible for two major programs that provide benefits based on disability: Social Security Disability Insurance (SSDI), which is based on prior work under Social Security, and Supplemental Security Income (SSI).  Under SSI, payments are made on the basis of financial need.
Social Security Disability Insurance (SSDI) is financed with Social Security taxes paid by workers, employers, and self-employed persons.  To be eligible for a Social Security benefit, the worker must earn sufficient credits based on taxable work to be "insured" for Social Security purposes.  Disability benefits are payable to blind or disabled workers, widow(er)s, or adults disabled since childhood, who are otherwise eligible.  The amount of the monthly disability benefit is based on the Social Security earnings record of the insured worker.
Supplemental Security Income (SSI) is a program financed through general revenues.  SSI disability benefits are payable to adults or children who are disabled or blind, have limited income and resources, meet the living arrangement requirements, and are otherwise eligible.  The monthly payment varies up to the maximum federal benefit rate, which may be supplemented by the State or decreased by countable income and resources. 
The medical eligibility requirements are the same for both programs but there are important differences that will determine which program you qualify for.

Medical Determination