The present thesis investigates the music and career of Jeffrey Lewis (born 1942). The thesis is broadly divided into three sections. First is an account of the composer's life, told mainly through an overview of his works, but also through a sketch of his early years in South Wales, his studies in Cardiff, Darmstadt, Kraków and Paris, his academic career in Leeds and Bangor, and his subsequent early retirement from academia. There follows a more detailed study of six works from the period 1978 - 1985, during which certain features of Lewis's musical language came to the fore, perhaps most notably a very individual and instantly recognisable use of modal language. After an Epilogue, the thesis concludes with an Appendix in the form of a Catalogue in which all Lewis's known compositions are listed, together with details of performances, broadcasts and recordings.Lewis's music often plays with our temporal expectations; the close interrelationship between texture, structure, harmony and melody, and its effect upon our perception of the passage of time, are explored in the main analyses. These are conducted partly by means of comparison with other works by Lewis or his contemporaries. Memoria is examined in relation to a similarly tranquil score, Naaotwá Lalá, by Giles Swayne. The following chapter discusses the extra-musical inspiration for Epitaph for Abelard and Heloise, whose relationship to Tableau is then explored in the next. The difficulties of creating a large-scale structure that unifies the work's various harmonic elements are also investigated. The analysis of Carmen Paschale considers it in relation to Lewis's other choral music, whilst the final analytical chapter compares and contrasts two three-movement works, the Piano Trio and the Fantasy for solo piano. Lewis's melodic writing in the Piano Trio is discussed in relation to that of James MacMillan, and the origins of the first movement of Fantasy in Oliver Knussen's Sonya's Lullaby are explored. In the Epilogue, the possible reasons for Lewis's current neglect are explored, various influences on Lewis's musical thinking are laid out, and his achievements are assessed.