The modern systems of verb endings found in Korean and Japanese have developed by incorporating certain auxiliary stems and delexicalized nouns, and many of these have etymologies that are shared by both languages. Arguments are presented that the mutually exclusive Middle Korean RETROSPECTIVE, EFFECTIVE (or aorist), and PROCESSIVE aspectual markers are bound auxiliaries attached to the verb stem. Moreover, the important infinitive ending is shown to have properties that indicate that it originated as a similar bound auxiliary. The modern Korean endings -ko, -ki, and -key are explained as complexes built on the effective marker plus other elements. Remarks are also made on the development of other formations in Korean and Japanese. The evidence continues to point to a prehistoric relationship between Korean and Japanese that is more intimate than can be demonstrated between either of those languages and any other language.