Conversion, the deliberate transfer of a word from one part of speech to another without any change in its form, is a typically English phenomenon, conditioned but not caused by the extensive wearing-off of word endings and weakening of inflections. It has typically been treated as a syntactic matter, since no new words are produced, and its investigation has been limited by the forms studied and the unsatisfactory models of word formation used for analysis. However, the process of conversion can also be approached through case grammar and a suitable transformational grammar. This approach does not have the flaws of previously used approaches, and may allow more insight into what really happens in the transfer process and how it occurs. Sixty-four references are supplied. (MSE)