To qualify for SSDI, you must have a medical condition that prevents you from performing substantial work and is expected to last at least 12 months or result in death. You must also have worked long enough and recently enough under Social Security to have accumulated the required work credits. The specific number of credits depends on your age when you became disabled. If you do not have sufficient work credits, you may qualify for Supplemental Security Income (SSI) instead, which has different eligibility requirements.