This article introduces a significant yet largely overlooked problem in the law of torts: causal underdetermination. This problem occurs when the causal inquiry of a but-for test produces not one but two results, which are contradictory. According to the first, the negligent defendant is the likely cause of the plaintiff's injury, whereas according to the second, she is not. The article explains why causal underdetermination has escaped the radar of tort scholars and is perceived by courts as lack of causation. It demonstrates that the current practice in cases of causal underdetermination might lead to erroneous decisions, absolving negligent defendants of tort liability even when the evidence suggests that they are in fact the likely cause of the plaintiff's injury. This, in turn, the article asserts, may not only lead to underdeterrence among potential defendants, but also encourage manipulative litigation strategy to escape liability in retrospect. The article then proposes solutions that contend with causal underdetermination and resolve the difficulties that the current practice entails.