Medical evidence is drawn from observation, is multifactorial, and relies on the laws of probability rather than a single cause, but, in law, finding causation between a wrongful act and harm is essential to the attribution of legal responsibility. These different perspectives often result in dissatisfaction for litigants, uncertainty for judges, and friction between health care and legal professionals. Carpal tunnel syndrome (CTS) provides an example: Popular notions suggest that CTS results from occupational arm or hand use, but medical factors range from congenital or acquired anatomic structure, age, sex, and body mass index, and perhaps also involving hormonal disorders, diabetes, pregnancy, and others. The law separately considers two separate components of causation: cause in fact (a cause-and-effect relationship exists) and proximate or legal cause (two events are so closely related that liability can be attached to the first event). Workers’ compensation systems are a genuine, no-fault form of insurance, and evaluators should be aware of the relevant thresholds and legal definitions for the jurisdiction in which they provide an opinion. The AMA Guides to the Evaluation of Permanent Impairment contains a large number of specific references and outlines the methodology to evaluate CTS, including both occupational and nonoccupational risk factors and assigning one of four levels of evidence that supports the conclusion.