This book details an investigation on the ability of laboratory rats to discriminate between stimuli that were the same or different. The ability to discriminate between similar and differing stimuli (i.e., a relative class concept) has been one of the oldest, and most predominantly employed, methods of cross-species intelligence comparisons. However, while many species have shown relative class concept learning ability, rats have not been shown to have this capacity. Most researchers agree that rats are likely to possess this ability, but the failure to empirically demonstrate the possession of the concept has been an enigma in the field of animal cognition. This manuscript details an attempt to demonstrate the laboratory rat has this capability and seeks to provide avenues for future research in this field.