The language property of error-detection ensures that the communications medium cannot transform a word of the language to another word of the language. In this paper we provide some insights on the notion of error-detection from a language theoretic point of view. We deÿne certain error-detecting properties of languages and codes including the notion of error-detection with ÿnite delay which is a natural extension of unique decodability with ÿnite delay. We obtain results about the error-detecting capabilities of regular and other languages, and of known classes of codes. Moreover, we consider the problem of estimating the optimal redundancy of inÿnite languages with the property of detecting errors of the deletion type.