This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book presents the framework to investigate reliability within the paradigm of classical test theory. It discusses the two fundamental concepts required in the application of Rasch measurement in language assessment research, i.e., unidimensionality and local independence. The book illustrates one of the most commonly used differential item functioning (DIF) detection methods, the Rasch-based DIF analysis method. It also discusses one of the most popular item response theory-based methods to analyze rater-mediated assessments. The book provides a didactic presentation of the multidimensional Rasch model. It describes a method for modeling growth that is called student growth percentile for longitudinal data, which is estimated by using the quantile regression method. The book explores the role of cross-validation in classification and regression tress modeling and the features of two cross-validation methods.