The present master’s thesis is an account of the semantic development of the two modal verbs CAN and MUST in the history of the English language. They were investigated through the use of the Helsinki Corpus of English Texts, where two genres were the focal point: historical and legal texts. By using approximately 100 examples of each modal, spread across Old English, Middle English and Early Modern English, the study presents the semantics of the modals at different points in time, and establishes how the meanings have changed throughout the history of the language. There is a focus on central and peripheral meanings with the modals, where a central meaning of the present day can be found as a peripheral meaning in older language, and other peripheral meanings may serve as an intermediate stage between two central meanings. MUST is found to convey the meaning of ‘permission’ and ‘obligation’ in Old English, ‘permission’ being the central in Old English, although ‘obligation’ is found as a close-to-central meaning as early as mid Old English. The ‘ability’ sense and the negated ‘permission’ sense may have functioned as the bridge between ‘permission’ and ‘obligation’. The development accelerates in Middle English, and by late Middle English we can be certain that the ‘permission’ sense is lost. The next step of development for MUST is the epistemic sense, which comes into use in Early Modern English.CAN has a full verb sense of ‘to know’ in Old English, but the ‘ability’ sense is at this stage present as well. The peripheral meanings that exist for CAN are very close to the meanings of ‘to know’ and ‘ability’, and it is therefore difficult to see a clear shift. The meanings glide into one another, which gives a continuum rather than a shift. The historical texts show earlier and different signs of development than the legal texts, but the difference is not as striking as hypothesized. However, the variety of senses found in the historical genre is great compared to legal texts, which implies that this is a more innovative genre. Historical texts seem to be an appropriate genre for investigating change and development because of the language’s closeness to spoken language, as opposed to legal texts, which are as far from colloquial as one can come.