For many years, scholars have investigated instructional design expertise and described the difficulty defining it. A lack of a clear definition, inclusive of primary components, poses a measurement problem for those seeking to evaluate the development of expertise. An overarching aim of this study is to gather evidence to support a definition of instructional design expertise (IDE) which includes knowledge, skills, and dispositions. Instructional design dispositions have not received much attention, but dispositions (e.g., adaptability, flexibility) are often described as distinguishing traits of expert designers. Existing ID competency instruments evaluate the perceived importance of knowledge and skills but are limited in tracking development past competency. They also do not adequately consider dispositions. The purpose of this research was to describe the development of the Instructional Design Dispositions and Expertise Index. Instrument development procedures included item generation, expert review, think-aloud sessions, and a small-scale item tryout. Over 200 designers agreed knowledge, skills, and dispositions are important components of expertise. Qualitative data corroborated quantitative findings further illuminating a relationship between these components and quality instructional design. Initial evidence of content and construct validity for the instrument is established. A validated expertise instrument would allow us to more fully understand and evaluate expertise and its development, which could inspire innovation in instructional design research, theory, and practice. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]