The main objective of this report was to provide an introduction to the subject of contaminant removal effectiveness. Existing literature in this subject area is limited, and tends to be very difficult for a newcomer to understand. In recent years, a number of parameters have been defined in order to quantify contaminant removal effectiveness, but not all authors have used the same names or symbols for similar parameters, or derived them in the same way. The usefulness and applicability of the various parameters has not been presented in a comprehensive way in a comparative format and, although the measurement of these parameters has been reported by several authors, there are few published summaries of the most suitable methods. Finally, none of the existing parameters provide a relative measure of contaminant removal effectiveness in the same way as air change efficiency, which provides a comparison with piston flow. Therefore, this report aims to show the origin of the concepts used, provide proofs of the basic formulae and suggests standard symbols and definitions. It also recommends methods of measurement with particular reference to difficulties and possible errors, and investigates the possibility of deriving a contaminant removal effectiveness parameter which will provide a measure of the performance of a ventilation system in removing a contaminant relative to some reference system.