Proposed lie-detection technology based on neuroscience poses significant challenges for the law. The law must respond to the science with an adequate understanding of such evidence, its significance, and its limitations. This paper makes three contributions toward those ends. First, it provides an account of the preliminary neuroscience research underlying this proposed evidence. Second, it discusses the nature and significance of such evidence, how such evidence would fit with legal practices and concepts, and its potential admissibility. Finally, it analyzes the constitutional protections that may limit the compelled production of such evidence.