This book develops an intuitive understanding of item response theory (IRT) principles through the use of graphical displays and analogies to psychological principles. The IRT psychometric method is especially useful to psychologists and other social scientists that are familiar with small scale cognitive and personality measures. Familiar concepts are used to explain various principles such as contemporary IRT models and estimation methods. Polytomous IRT models are given central coverage since many psychological tests use rating scales. Applications in various research areas are described. This book is especially popular among graduate students and practitioners who need to understand the contemporary psychometric foundations of the tests that they administer and/or use to analyze IRT scales in their research. It is an ideal book for clinical, industrial, counseling, educational, and behavioral medicine professionals and students familiar with classical testing principles. Exposure to material covered in first-year graduate statistics courses is helpful. All symbols and equations are thoroughly explained verbally and graphically.