This special issue focuses on an array of studies conducted using the Kaufman Test of Educational Achievement–Third Edition (KTEA-3) error analysis system. These studies, based on KTEA-3 standardization and validation data with normal and clinical samples, were conducted to provide greater understanding of the kinds of errors students make in reading, writing, math, and oral language. This introduction provides a brief history of the error analysis system and outlines the organization of the special issue, which features commentaries on the articles by experts in the field. The themes throughout the special issue are patterns of errors made by students and the educational implications of these patterns.