The purpose of this work is to examine the various ways in which modality is expressed in English. Firstly, the fundamental concept 'possible worlds' and a view of modality as 'relativization 1 is proposed. The way in which the concept of modality is related to utterance functions is then examined and the perspective of linguistics on modality is discussed. Chapter 3 provides an analysis of the English modals: it is argued that purely syntactic accounts have severe limitations and an 'atomistic' semantic analysis is proposed instead which makes it possible to relate the modals to non-auxiliary modal expressions. This analysis is extended in Chapter 4 to cover quasi-auxiliary modal expressions, adjectival, participial and nominal modal expressions, and modal adverbs and lexical verbs, which are all assessed in terms of a scale of "formal explicitness' and the type of modality they may be used to express. An explanation is offered of why such a wide range of modal expressions is available in English and why the modals are so often taken to be more 'central* to the expression of modality than other modal expressions. In Chapter 5 it is further argued that tense, IF-clauses and questions may also be regarded as ways of expressing modality. In Chapter 6 it is shown that many of the distinctions established in Chapters 3» ^ and 5 are reflected in the way modal expressions are used to convey politeness, which suggests that such syntactico-semantic categories may well be pragmatically motivated. Chapter 7 traces the development of modal expressions in the speech of children up to the age of 12, and it is seen that the various changes which occur appear to be in line with what is known of cognitive development in conjunction with the nature of modal linguistic devices as analysed in Chapters 3. 4 and 5.