There is an increasing need for assessment instruments to evaluate pedagogical techniques and curricular reforms in science and engineering. This paper presents the development of an assessment tool designed to measure students' understanding of fundamental concepts in electromagnetics. The tool is termed the electromagnetics concept inventory (EMCI). Although primarily intended for junior-level electromagnetics courses in electrical engineering departments, the EMCI can also be used in a variety of undergraduate and graduate electromagnetics-related courses in engineering and physics departments. The instrument may be used for several different purposes and can be administered in different ways, but the most important purpose is to assess students' performance and evaluate the effectiveness of instruction, usually as the "gain" between the course "pre-test" and "post-test" scores. The EMCI Version 1.0 is composed of three exams: EMCI-Fields, EMCI-Waves, and EMCI-Fields & Waves, to allow instructors to target specific knowledge areas. The questions on the exams focus on the core concepts of the material while requiring little or no computation.