Cognitive Diagnostic Assessment (CDA) is a type of educational assessment that is designed to measure specific knowledge structures and processing skills in students so as to provide information about their cognitive strengths and weaknesses (Leighton & Gierl, 2007). CDA has been instrumental in turning the attention of practitioners to more diagnostic, descriptive, and fine-grained levels of feedback. Different statistical, psychometric models, called Cognitive Diagnostic Models (CDMs), have been proposed to extract this kind of information from both diagnostically and non-diagnostically designed tests. These models provide two sets of information to the test users: information on mastery/non-mastery patterns of sub-skills for testtakers and information on the diagnostic power of test items. Due to its novelty and relative complexity of its procedures, cognitive diagnostic assessment is still far from achieving its proper place in educational assessment. This paper aims at providing an easy-to-grasp account of CDA's theoretical foundation and its procedures of test analysis. The present paper first focuses on what and why of CDA in education and second language acquisition. In this part, theoretical underpinnings of CDA, psychometric models of practicing the analyses, model selection, and studies in SLA are presented. The second section presents how these foundations are put into practice in a stepwise manner. Four main steps are delineated in conducting a CDA analysis. The procedural steps are then exemplified using real data for analysis. The paper concludes with an account of the limitations and untapped areas in CDA.