Through a project funded by the National Science Foundation, Horizon Research has been developing assessment items for students (in the process, compiling item-writing principles from several sources and adding their own). In this article, the authors share what they have learned about writing items that reveal student understanding, including best practices and concrete examples of applying them. Although most of the examples given are multiple-choice items, the principles are generally applicable to open-ended questions as well. (Contains 3 figures and 1 resource.)