This thesis is an exploration of ways in which the relationship between the 'how' and 'what' of textual meaning and variation can be theorised. Its major aim is to develop a functional theory of language which specifies register in terms of contextual and linguistic features, and which postulates a discursive relationship between the two. It aims, therefore, to theorise, in a more developed way than has so far been done, a functional linguistic account of register and a theoretical understanding of the relationship between text, context, language function and linguistic form. To this end I develop a conceptual model of contextualisation and relate this to a Cline of Register by which the progression of textual meaning, discursive variation and functional complexity can be represented