Biometric systems are being used in more places and on a larger scale than ever before. As these systems mature, it is vital to ensure the practitioners responsible for development and deployment, have a strong understanding of the fundamentals of tuning biometric systems. The focus of biometric research over the past four decades has typically been on the bottom line: driving down system-wide error rates. In doing so, powerful recognition algorithms have been developed for a number of biometric modalities. These algorithms operate exceedingly well under test conditions.