This study uses the 2.7 million word Corpus of Early English Correspondence to trace the development of the modal auxiliary MUST and its two main meanings, ’personal obligation’ and ’logical necessity’ through the span of the corpus, from the early 15th century to the late 17th. The approach is sociolinguistic, with particular reference to gender differences in the use of the auxiliary. The main finding is that MUST increased steadily in frequency during the Early Modern English period. This increase is found both in the language of men and women, with men leading in the use of MUST in the 15th century and women reaching and overtaking their level of usage by the 17th century. The use of MUST also increases in both meanings, but the ’logical necessity’ meaning is clearly less frequent than the ’personal obligation’ sense. The rise of ’logical necessity’ seems to stem from educated men, from whom its use spreads to other men and to educated women.