Roughly two in three initial applications for Social Security disability benefits are denied, and a large share of those denials are reversed later by the same agency applying the same rules. This article explains why, by walking the decision the government actually makes: a five-step sequential evaluation that asks about work activity, medical severity, listed impairments, past work, and finally whether any work at all exists that the claimant could do. It explains the two programs people confuse constantly, the insurance-based SSDI system and the needs-based SSI system, and why the distinction determines back pay, health coverage, and household eligibility. It covers residual functional capacity as the hinge concept, the medical-vocational grid upheld in Heckler v. Campbell, and the vocational expert testimony that decides most contested cases after Biestek v. Berryhill. The second half is the appeals architecture — reconsideration, the ALJ hearing where most awards happen, the Appeals Council, and federal court review for substantial evidence — plus the money questions of onset date, the five-month waiting period, Medicare eligibility, offsets, and how representatives are paid. It closes with the traps that sink otherwise strong claims.