Workers' compensation is the oldest social insurance program in the United States and the one most lawyers understand least. This article explains the bargain at its center: workers gave up the right to sue for negligence and to recover pain and suffering, and employers gave up the common law defenses that had made recovery nearly impossible, in exchange for prompt, certain, no-fault benefits paid on a schedule. It walks the compensability question that decides most disputes, including the two-part arising-out-of and in-the-course-of test, the going and coming rule and its many exceptions, and the treatment of occupational disease, cumulative trauma, and psychological injury. It covers the benefit structure from medical care through temporary and permanent disability and death benefits, and explains why impairment ratings and average weekly wage computation generate more litigation than causation does. It then takes up the exceptions that let a case escape the exclusive remedy, the third-party action and the employer's lien that follows it, the misclassification and statutory employer questions that decide who is covered at all, and the Medicare Secondary Payer obligations that complicate every settlement. It closes with the system's real failures and the reform debate around them.