Assessment of student learning outcomes can be a powerful tool for improvement of instruction when a scientific approach is taken; unfortunately, many educators do not take full advantage of this approach. This article examines benefits of taking a scientific approach to critical thinking assessment and proposes guidelines for planning, conducting, and using assessment research. Specifically, we discuss study design options and strategies for improving the quality of assessment data and the use of such data to improve critical thinking instruction in programs. Examples of practices from three programs that have conducted considerable critical thinking assessment illustrate use of the guidelines. Using scientific assessment to develop evidence-based educational practices shows great promise in contributing to the scholarship of teaching critical thinking. (Contains 2 tables.)