The main objective of this report was to provide a concise introduction into the subject of air change efficiency. Existing literature in this subject area is extensive, but it tends to be very detailed and is difficult for a newcomer to understand. Different authors also use different symbols and/or different definitions for the same concepts, which tends to confuse the reader. Little has been produced covering the basic ideas and concepts behind some of the terms used. Therefore this report aims to show the origins of the concepts used, provide proofs of the basic formulae and suggests standard symbols and definitions. Sandberg and Skaret differentiate between the terms air change efficiency and ventilation efficiency. Air change efficiency is a measure of how effectively the air present in a room is replaced by fresh air from the ventilation system whereas ventilation efficiency is a measure of how quickly a contaminant is removed from the room. This report covers only air change efficiency and related concepts. It should also be noted that the theory described in this report assumes a mechanically ventilated, airtight room where all the air enters and leaves via designated inlet and exhaust ducts.