Language tests play pivotal roles in education, research on learning, and gate-keeping decisions. The central concern for language testing professionals is how to investigate whether or not tests are appropriate for their intended purposes. This book introduces an argument-based validity framework to help with the design of research that investigates the validity of language test interpretation and use. The book presents the principal concepts and technical terms, then shows how they can be implemented successfully in practice through a variety of validation studies. It also demonstrates how argument-based validity intersects with technology in language testing research and highlights the use of validity argument for identifying research questions and interpreting the results of validation research. Use of the framework helps researchers in language testing to communicate clearly and consistently about technical issues with each other and with researchers of other types of tests.