This Article presents the results of my unique study of 800 criminal cases addressing neuroscience evidence over the past two decades (1992–2012). Many legal scholars have theorized about the impact of neuroscience evidence on the criminal law, but this is the first empirical study of its kind to systematically investigate how courts assess the mitigating and aggravating strength of such evidence. My analysis reveals that neuroscience evidence is usually offered to mitigate punishments in the way that traditional criminal law has always allowed, especially in the penalty phase of death penalty trials. This finding controverts the popular image of neuroscience evidence as a double-edged sword—one that will either get defendants off the hook altogether or unfairly brand them as posing a future danger to society. To the contrary, my study indicates that neuroscience evidence is typically introduced for a well-established legal purpose—to provide fact-finders with more complete, reliable, and precise information when determining a defendant’s fate. My study also shows that courts accept neuroscience evidence for this purpose, and in fact expect attorneys to raise this evidence when possible on behalf of their clients. This expectation is so entrenched that courts are willing to grant defendants their “ineffective assistance of counsel” claims when attorneys fail to pursue this mitigating evidence. Meanwhile, my study also reveals that the potential future danger posed by defendants is rarely a facet of cases involving neuroscience evidence—again contradicting the myth of the double-edged sword. The cases that do address future danger, however, offer fascinating insight into the complex legal issues raised by neuroscience evidence. As courts continue to embrace neuroscience tools and techniques, the empirical data collected in my study provide a foundation for discussions regarding the use of neuroscience evidence in criminal cases. The findings presented in this Article will ensure that those discussions are grounded in fact rather than hyperbole.