Diagnostic testing has long been valued by language testing researchers and practitioners for its ability to inform both teaching and learning. However, the number of well-developed diagnostic language tests is relatively low, most likely due to the difficulty in constructing a diagnostic test that incorporates a wide enough range of language skills. In recent years, the advancement of technologies has allowed for the development of several large-scale, computer-based diagnostic language tests, as well as more sophisticated measurement methods to conduct diagnosis. This article first provides the theoretical ground for how diagnosis has been conceptualized in language testing over the past 60 years. Then, several current approaches to operationalizing diagnosis in second and foreign language assessment, including both tests and methods, are reviewed. The article concludes with reflections on diagnostic testing in second and foreign language assessment at its current state, as well as recommendations for its future directions.