This publication introduces the Diagnose-and-Supplement System of Basic Skills (DASOBS), an online–offline system for academically low-achievement students from grades 1 to 10 in the Republic of Korea. It outlines the purpose, features, and history of DASOBS and describes the pipeline, content, and operations of the system. DASOBS, which involves form-based adaptive testing, is administered four times a year, with supplemental materials provided so that students can study subjects and lessons according to their achievement levels. This publication also presents policy implications of DASOBS in the education sector. It will be useful for policymakers, educators, and other stakeholders, especially those seeking to increase targeted support for academically underachieving students.