If one understands the nature of linguistic structure, one will know what design features an adequate machine translation system must have. To put it the other way around, it is futile to attempt the construction of a machine translation system without a knowledge of what the structure of language is like. This principle means that if someone wants to construct a machine translation system, the most important thing he must do is to understand the structure of language. Any MT system, whether by conscious intention on the part of its creators or not, is based upon some view of the nature of linguistic structure. By making explicit the underlying theory for various MT systems which have been proposed we can determine whether or not they are adequate. Similarly, by observing linguistic phenomena we can determine what properties an adequate theory of language must have, and such determination will show what features an MT system must have in order to be adequate. It can be shown that some of the approaches to MT now being pursued must necessarily fail because their underlying linguistic theories are inadequate to account for various well-known linguistic phenomena.