Lady Justice, with her blindfold and gilded scales, is the image is evoked to illustrate the standard used in civil trials: the plaintiff must prove his or her case by a preponderance of the evidence, equating “preponderance” with a greater than 50% chance of the plaintiff’s claims being true. The legal doctrine res ipsa loquitur employs a similar metric when it comes to allowing an event that does not ordinarily occur in the absence of negligence to serve as circumstantial evidence of the plaintiff’s claim. This article describes the current application of res ipsa in Pennsylvania, focusing particularly on medical malpractice cases.