“Psychometrics” refers to the scientific discipline that combines psychological inquiry with quantitative measurement. Though psychometric theory and practice pertain to all aspects of measurement, in the current context, psychometric approaches to expertise pertain to the measurement and prediction of individual differences and group differences (e.g., by gender, age) and, in particular, high levels of proficiency including expertise and expert performance. The scientific study of expertise involves several important psychometric considerations, such as reliability and validity of measurements, both at the level of predictors (e.g., in terms of developing aptitude measures that can predict which individuals will develop expert levels of performance), and at the level of criteria (the performance measures themselves). The authors discuss these basic aspects of psychometric theory first, and then they will provide an illustration of psychometric studies that focus on the prediction of expert performance in the context of tasks that involve the development and expression of perceptual-motor skills, and tasks that involve predominantly cognitive/intellectual expertise. Finally, they will discuss challenges for future investigations.