The soul of America's civil and criminal justice systems is the ability of jurors and judges to accurately determine the facts of a dispute. This invariably implicates the credibility of witnesses. In making credibility determinations, jurors and judges necessarily decide the accuracy of witnesses' memories and the effect of the witnesses' demeanor on their credibility. Almost all jurisdictions' pattern jury instructions about witness credibility explain nothing about how a witness's memories for events and conversations work-and how startlingly fallible memories actually are. They simply instruct the jurors to consider the witness's "memory" with no additional guidance. Similarly, the same pattern jury instructions on demeanor seldom do more than ask jurors to speculate about a witness's demeanor by instructing them to merely observe "the manner of the witness" while testifying. Yet, thousands of cognitive psychological studies have provided major insights into witness memory and demeanor. The resulting cognitive psychological principles that are now widely accepted as the gold standard about witness memory and demeanor are often contrary to what jurors intuitively, but wrongly, believe. Most jurors believe that memory works like a video camera that can perfectly recall the details of past events. Rather, memory is more like a Wikipedia page where you can go in and change it, but so can everyone else. Memories are so malleable, numerous, diverse, and innocuous that post-event information alters them, at times in very dramatic ways. Memories can be distorted, contaminated, and even, with modest cues, falsely imagined. For example, an extremely small universe of people have highly superior autobiographical memory ("HSAM"). They can recall past details (like the color of the shirt they were wearing on August 1, 1995)from memory almost as well as a video camera. HSAM individuals' memories are not infallible, however. In one study, HSAM participantsfa lsely remembered seeing news film clips of United Flight 93 crashing in afield in Pennsylvania on September 11, 2001. No such film exists. Thus, no group that is free from memory distortions has ever been discovered. In one interesting study, students on a college campus were asked to either perform or imagine certain normal and bizarre actions: (1) check the Pepsi machine for change; and (2) propose marriage to the Pepsi machine. Two weeks later, the students were tested and demonstrated substantial imagination inflation leading to false recognition of whether they performed or imagined the actions. Few legal principles are more deeply embedded in American jurisprudence than the importance of demeanor evidence in deciding witness credibility. Historically, demeanor evidence is one of the premises for the need for live testimony, the rule against hearsay, and the right of confrontation under the Sixth Amendment to the United States Constitution. Yet, cognitive psychological studies have consistently established that the typical cultural cues that jurors rely on, including averting eye contact, a furrowed brow, a trembling hand, and stammering speech, for example, have little or nothing to do with a witness's truthfulness. Also, jurors all too often wrongly assume that there is a strong correlation between a witness's confidence and the accuracy of that witness's testimony. Studies have determined that jurors'perceptions of witness confidence are more important in determining credibility than the witness's consistency or inconsistency. Another series of studies indicate that, in reality, demeanor evidence predicts witness truthfulness about as accurately as a coin flip. Once the fact-finder makes credibility determinations, it is nearly impossible to overturn those decisions on post-trial motions or appeal. The secrecy with which credibility determinations are made promotes the legitimacy of factfinding, but it also shrouds its countless failings. Despite years of overwhelming consensus among cognitive psychology scholars and numerous warnings from thoughtful members of the legal academy, judges have done virtually nothing to identify or to begin trying to solve this serious problem. The one exception is eyewitness identification of suspects in criminal cases, where several state supreme courts have relied heavily on cognitive psychological research to craft better science-based specialized jury instructions. This Article examines and analyzes the often amazing and illuminating cognitive psychological research on memory and demeanor. It concludes with a Proposed Model Plain English Witness Credibility Instruction that synthesizes and incorporates much of this remarkable research.