Adaptive online quizzing is a technique that allows test items to be chosen based on the test-taker’s response in a web-based environment. It is demonstrated especially useful for first and second language vocabulary assessment because of the statistical reliability, efficiency, and engagement. Adaptive vocabulary tests and quizzes, built on an adaptive online quizzing system comprising a calibrated item bank, an adaptive computer programme, and online platforms, offer advantages and superior features over fixed-item tests. These adaptive tests effectively assess complex constructs of word knowledge and enhance online vocabulary learning. Despite some challenges in development and application, adaptive online quizzing will continue to enhance vocabulary assessment by measuring more aspects of word knowledge and integrating quiz results with offline vocabulary teaching methods.