"Language Assessment Quarterly," Volume 6, Issue 3, 2009, is a special issue titled "Cognitive Diagnosis and Q-Matrices in Language Assessment." This special issue is a textbook for cognitive diagnosis, one that language testers can study for many years to come. This article presents the author's reaction on why cognitive diagnosis is necessary. The author talks about the emergence and application of cognitive diagnosis approaches (CDA) from a historical perspective. (Contains 7 footnotes.)