This article describes special aspects of addressing and defining substantial medical evidence, causation, and apportionment in the California Workers' Compensation system. Substantial medical evidence is framed in terms of reasonable medical probability, and the opinion must be based on fact and not be speculative. The issue of whether the injury occurred in the course of employment is left to the Trier of Fact (WCAB judge). The issue of arising out of employment is a medical issue left to the physician. Apportionment applies to both the industrial and nonindustrial cause of the disability.