This paper discusses some theoretical aspects of contrastive phonetics. A fundamental problem in contrasting the sound structure of two languages is the question of equivalence between the two sound systems. There are four possible criteria: (1) similar spelling; (2) similar phonetic description and transcription; (3) use of phonological criteria; and (4) perceptual similarity. K. Wiik's phonetic contrasting and substitution tests involve an acoustic comparison of physical phones combined with a systematic approach to their phonological relationships, and can be used to detect interference between units which could hardly be equated on the basis of phonological structures. Because of differing learning strategies and the hierarchy of errors associated with interference, results of contrastive phonetics cannot be considered final. And finally, the relationship of contrastive phonetics to language teaching problems must be viewed in the light of the fact that, while contrastive phonetics involves a kind of error analysis, "errors" as such are not the object of study in this kind of analysis. Rather, it seeks to determine how native speakers react to various kinds of deviations from phonetic habits of their own. Therefore, the criteria used by language teachers to evaluate pronunciation are different from those used by contrastive phonetic analysis. (AM)