Courts have, traditionally, required the plaintiff to prove to the requisite standard of proof that causation exists. This monograph addresses what has, to date, been a modest reform toward permitting recovery based on a probability that causation exists, discounting the damages awarded by the probability of causation. The central purpose of our inquiry is to employ a comparative methodology to better understand how different legal systems respond to causal uncertainty and to examine why, how and in what situations rules of proportional liability can promote the goals of tort law.