Psychological and educational tests are important tools of the behavioral sciences. Development, analysis, and application of tests are studied within psychometrics. Since modern psychometrics was founded, both theory and practice of psychometrics have developed prosperously: Tests are continuously developed, classical theories are extended and subsumed under modern theories, and new applications are designed. Unfortunately, test construction and test theory are developing rather separately. Test constructors are often badly informed about modern theories, and test theorists are badly informed about test construction. This book tries to bridge the gap between theory and practice by discussing both test construction and test theory. Gideon J. Mellenbergh is emeritus professor of Psychological Methods at the University of Amsterdam, former director of the Interuniversity Graduate School of Psychometrics and Sociometrics, and emeritus member of the Royal Netherlands Academy of Arts and Sciences. His research interests are in the construction of psychological and educational tests, psychometric decision-making, measurement invariance, and the analysis of psychometrical concepts. His teaching was on a large number of methodological topics (designs, measurement, and statistical analysis) for audiences that vary from freshmen to dissertation students. Recently, he teaches courses on methodological consultancy for research master and dissertation students. He published in international methodological journals (e.g., Applied Psychological Measurement, Journal of Educational Measurement, Multivariate Behavioral Research, Psychological Methods, and Psychometrika), contributed to methodological books, and (co-) supervised 88 PhD students who successfully defended their thesis.