This Handbook, with 45 chapters written by the world’s leading scholars in second language acquisition (SLA) and language testing, dives into the important interface between SLA and language testing: shared ground where researchers seek to measure second language performance to better understand how people learn their second languages. The Handbook also reviews how to best measure and evaluate the second language (L2) learners’ personal characteristics, backgrounds, and learning contexts to better understand their L2 learning trajectories. Taking a transdisciplinary approach to research, the book builds upon recent theorizing and measurement principles from the fields of applied linguistics, cognitive science, psychology, psycholinguistics, psychometrics, educational measurement, and social psychology. The Handbook is divided into six key sections: (1) Assessment concepts for SLA researchers, (2) Building instruments for SLA research, (3) Measuring individual differences, (4) Measuring language development, (5) Testing specific populations, and (6) Measurement principles for SLA researchers.