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 describes one of the most popular item-response theory-based methods to analyze rater-mediated assessments. It deals with a brief discussion of the differences between single and multiple linear regression and a full discussion on the assumptions that must be fulfilled before commencing analysis. The book presents a method for modeling growth that is called student growth percentile for longitudinal data and is estimated by using the quantile regression method. Aryadoust introduces the evolutionary algorithm-based symbolic regression method and showcases its application in reading assessment.