Validation is an important enterprise especially when the test to be validated is a high stakes whose results bring about life changing implications for the candidates. A fair test needs to be neutral when it comes to construct irrelevant factors such as demographical variables like gender and field of study, etc. This book reviews and discusses traditional and current views of validity and explains the procedures, techniques, and methods to evaluate it. It provides a comprehensive examination of all key concepts in test validation process, test fairness, along with systematic and user friendly treatment of doing DIF for multiple choice tests of English language proficiency. After exploring the basic considerations underlying the theoretical and practical investigation of differential item functioning and explaining various DIF detection methods and techniques, it offers testing and assessment practitioners step-by-step guidance in using three-step Logistic Regression (LR). This Book is recommended as an authoritative complement to graduate and under graduate teacher-training courses in language testing and assessment, and professional practitioners in this field.