A key component of research within information systems is the use of valid instruments to measure numerous aspects of technology, organizations, and people. Validated instruments exist for many different variables and more are available all the time. Knowledge of a particular domain is an integral part of working competently, effectively. and successfully within that domain (N.M. Degele. World Futures, 50, 743-755, 1997). Therefore, this paper will describe the rationale for the development of an instrument to measure the information systems knowledge of individuals. This rationale iv tooted in several streams of research, such as absorptive capacity and certainty of knowledge, and focuses on three main questions: why measure knowledge of information systems, how to measure this knowledge, and what to actually measure. The definitions of knowledge and information systems knowledge are developed and refitted through an analysis of the literature covering the philosophical, psychological, and educational aspects of knowledge. The resulting models of knowledge and specifically information systems knowledge are then applied to research streams within the information systems discipline as well as practitioner-oriented needs.