Although he describes himself as a 'modernist', tradition has been an integral aspect of the music of James MacMillan from the beginning of his compositional career. Three traditions in particular permeate his works: the cultural tradition of his native Scotland; the religious tradition of the Catholic Church; and the tradition of music from past models to the present day. These three traditions and their relationship to the present are explored in depth in this thesis, which argues that MacMillan should be termed a 'retrospective modernist', given the emphasis he places on the relationship between past traditions and the present. Part I examines MacMillan's political and cultural retrospective modernism, initially in the general context of autonomous and political music, and then more specifically in relation to Liberation Theology, while the remainder explores MacMillan's complex relationship with Scotland, past and present. Part II discusses religious retrospective modernism, comparing MacMillan with Wagner and Bach. The theological implications of Wagner's Tristan und Isolde are discussed in conjunction with The Sacrifice and St John Passion, and the following chapter addresses three liturgical issues concerning both MacMillan and Bach: musical settings of the Christian Passion narrative; the composition of music for congregational participation; and the practice of recycling music in mass settings. Part III continues with this topic of musical recycling and quotation in MacMillan's works, first in comparison with Mahler, and then extending to consider the technique of polystylism in the music of Ives, Berio, Schnittke and Maxwell Davies, all of whom have been significant influences on MacMillan. Finally, an in-depth examination of the tension between the concepts of tradition and modernism concludes the study. While some modernists see this tension as irreconcilable, MacMillan considers it to be a positive, creative tension. Issues relating to high modernism, antimodernism, postmodernism, pluralist modernism and finally retrospective modernism are discussed here, demonstrating why the latter is the most appropriate term to describe the music MacMillan composed in the period 1982-2010.