At The Queen's University of Belfast there has been, for some twenty years, a continuing program of research into scavenging flow in two-stroke cycle engines, recently using a single-cycle gas scavenging apparatus. This apparatus has been demonstrated to give accurate assessments of the scavenging efficiencies of such engine cylinders. The apparatus utilises a constant volume, isothermal flow process in the experimental simulation of the scavenging flow and, as many of the classic theories of scavenging are similarly formulated, this provides a unique opportunity to compare theory and experiment on the basis of equality of procedure. This paper presents experimental data for the scavenging characteristics of uniflow-, loop-and cross-scavenged two-stroke engine cylinders and compares the measurements with the classic theories of scavenging as presented by others in the literature. It is shown that none of the classic theories of scavenging are able to correlate with the experimental facts over the full range of scavenge ratios. A model of scavenging is proposed which does fit the experimental facts in all cases and this theoretical model is easily solved on a desk-top micro-computer; a copy of the actual computer program, in BASIC, is provided for use by the reader. So that others may attempt to correlate their theories of scavenging with experiment, the author has included, in an appendix, the actual numeric values of the measured scavenging efficiency-scavenge ratio characteristics for uniflow-, cross-, and loop-scavenged engine cylinders.