Linguistic input probably has to be comprehensible to the learner if it is to serve as data for second language acquisition. It is widely assumed that input becomes comprehensible through the speech modifications of native speakers addressing non-native speakers of the target language. Recent research on native speaker/non-native speaker conversation suggests, however, that modifications of the interactional structure of conversation are more important in this regard. They are more extensive and more consistently observed than input modifications, and often occur when the latter do not. Fifteen devices for the modification of interaction are described. They are of three kinds: strategies, which serve to avoid conversational trouble, tactics, which are used to repair the discourse when trouble occurs, and strategies and tactics, devices which serve both functions.