Market-share liability has been one of the most controversial doctrines in tort law, with a strong plurality of courts rejecting the doctrine on the ground that it radically departs from the fundamental tort principle of causation. Courts that have adopted this liability rule, though, believe they are adhering to the principle of causation. In the first case to adopt market-share liability, the California Supreme Court claimed that the liability rule is grounded upon an extension of alternative liability, a doctrine that has been accepted by virtually all jurisdictions. The court never adequately explained how alternative liability can be modified to yield market-share liability, and the only explanation provided by torts scholars involves redefining the tort right to permit compensation for tortious risk, conditional upon the occurrence of injury, rather than for the injury itself. However, courts do not conceptualize the tort right in these terms, for otherwise the doctrine of market-share liability would be uncontroversial. As this Article shows, market-share liability can be derived from alternative liability in a manner that neither redefines the tort right nor departs from the principle of causation. Alternative liability permits the plaintiff to prove causation against the group of defendants. This characterization of the causal rule has been recognized by some torts scholars, but has never been justified. The Article shows that evidential grouping is a defensible principle implicit in numerous cases involving analogous causal problems, including the asbestos cases. Evidential grouping not only explains the doctrine of alternative liability, it shows how a modification of that liability rule yields market-share liability largely for reasons given by the California Supreme Court. This conceptualization of alternative liability and market-share liability also explains the otherwise puzzling liability rule adopted by courts in the asbestos cases. Due to this doctrinal unity, the widespread acceptance of alternative liability should make market-share liability more widely acceptable.