Psychometrics involves the development and evaluation of mathematical models that connect the observable behaviors of persons to attributes. Because psychometric models produce numeric quantities that are typically assigned to persons for some purpose, psychometrics is sometimes characterized as a scientific discipline concerned with the practice of psychological measurement. The historical roots of psychometrics come from an interest in the quantitative study of individual differences. Technological advances through the first quarter of the twenty-first century have led to the development and evaluation of increasingly complex and specialized mathematical models. There can be significant syntactical barriers to understanding these models, making it easy to lose sight of philosophical questions and debates about the meaning and interpretation of the latent variables these models contain. In this entry, these debates are brought to the surface and future possibilities for the field of psychometrics as it evolves are considered.