Unemployment insurance is the oldest and least understood social insurance program in the United States, and the single question it turns on is almost never the one people expect. This article explains how eligibility is actually decided: not by whether a separation was fair, but by a two-part statutory test asking whether the worker quit with good cause attributable to the employer or was discharged for misconduct connected with the work. It traces the federal-state structure that produces fifty different programs under one tax statute, the monetary eligibility rules that exclude many low-wage and part-time workers before any separation question is reached, and the continuing requirements to be able, available, and actively seeking work. It covers the employer's side — experience rating, protest deadlines, and the reasons employers contest claims that cost them little — and the appeal hearing, which is a genuine evidentiary proceeding that most claimants attend unprepared. The final sections address severance, partial unemployment, misclassification, overpayments and fraud penalties, and the interaction with disability, discrimination, and severance-agreement claims. A running hypothetical shows how the same separation produces opposite outcomes depending on facts nobody documented at the time.