The goal of human factors engineering is to optimize the relationship between humans and systems by studying human behavior, abilities, and limitations and using this knowledge to design systems for safe and effective human use. With the assumption that the human component of any system will inevitably produce errors, human factors engineers design systems and human/machine interfaces that are robust enough to reduce error rates and the effect of the inevitable error within the system. In this article, we review the extent and nature of medical error and then discuss human factors engineering tools that have potential applicability. These tools include taxonomies of human and system error and error data collection and analysis methods. Finally, we describe studies that have examined medical error, and on the basis of these studies, present conclusions about how human factors engineering can significantly reduce medical errors and their effects.