In the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides), Fifth Edition, apportionment refers to the “distribution or allocation of causation among multiple factors that caused or significantly contributed to the injury or disease and existing impairment,” and causation is the process of determining “an identifiable factor (eg, accident or exposure to hazards) that results in a medically identifiable condition.” Causality assessment requires a clear understanding and discussion of causal relationships; apportionment analysis refers to the extent to which factors may have contributed to a particular effect or impairment. A causal relationship requires three elements—a cause, an effect, and a specific relationship between them—the absence of any one of which disqualifies causality. Apportionment is an estimate of the degree to which each of various medically probable occupational or nonoccupational factors contributed to a particular impairment. The unique legal standards of compensability and apportionment vary by locality, and the apportionment of disability involves assessing the functional effects of different injuries over time. Evaluators should consider the effect of using the Combined Values Chart on the final assessment and must carefully distinguish factors that are combined vs those that are added or subtracted. A sidebar discusses important changes to the State of California's Workers’ Compensation.