Brigham Young University's Center for Instructional Design (CID) creates online courses and multimedia instructional applications for university faculty. This paper asserts that including usability testing as a part of evaluation improves the quality and effectiveness of computer-mediated instruction. The paper describes the fundamental purpose and functions of usability testing, and also distinguishes between different forms of evaluation: accessibility, quality assurance (QA), usability and implementation. Through a detailed case study, we describe how usability testing improved the quality of a computer-based chemistry course and facilitated a clearer analysis of the learning effectiveness of this course.