Curriculum-based measurement is a system for monitoring students' progress and formatively evaluating instruction backed by 25 years of validation research. Most of this research has been conducted in elementary schools. In middle and high school classrooms, where there is an emphasis on mastering content knowledge, elementary-level measurements are not viable. This article describes the development and validation of a curriculum-based measurement using a maze format, in which specific words are eliminated from sentences and students select from several options to fill in each omitted word. Unlike traditional maze tasks in which words are deleted with attention to grammar and syntax, a concept maze is used in which the target words represent key attributes of core concepts from the content domain. Data from a pilot study are presented along with several options for teachers to obtain data that will (a) inform them which students should receive supplemental instruction and (b) identify particular attributes that require further development. (Contains 5 figures.)