In the mid-1990s, the first North American specification for carboxylate-inhibited engine coolant that was accompanied by a recommended change interval of 5 years/ 100 000 miles was published. Others followed. Part of the testing protocol in these specifications require prospective suppliers to evaluate and report the compatibility of proposed formulations with previous, conventionally inhibited engine coolants, in particular widely referenced and manufactured formulations. Industry experience with the test methods required by these original equipment manufacturers (OEMs) has been the root cause of heated debate in the industry as to the definition of “compatibility” in light of the tremendous push of “all Makes All models” coolants. This paper compiles the compatibility tests that are, or have been, specified by OEM specifications and also reports research work performed by other companies in an attempt to identify possible problematic behaviors. Tests of intentionally mixed coolants are modified from standard OEM, ASTM, and JIS methods. Some test methods evaluate the effect of mixed coolants on unpassivated metal surfaces while others seek to assist in understanding of the effect of mixed coolants in systems that have been operated for some time on a single coolant technology (the “top up” environment).