This study is concerned with the style (as opposed to the function) of English vocal polyphony during the period ca.1525 - ca.1575. It focusses on the sacred works of Thomas tallis (ca.1505-1585); but the aim is as much to come nearer to a full understanding of mid-Tudor stylistic evolution as it is to define the place of Tallis' music within the broad context. Tallis is therefore viewed both as an individual and as a representative of his time, and his music is assessed analytically rather than critically. The study takes as its premise the view that English music of the period is too often examined according to useful but unrealistic categories - sacred or secular, Catholic or Protestant, institutional or domestic, Henrician, Edwardian, Marian or Elizabethan - and that a sympathetic understanding of the evolution can only be reached when the period is considered in a broad, comprehensive and chronological sweep. It also argues that the evolution was largely stimulated by influence from abroad (the place of foreign music and musicians in mid-Tudor England is studied in detail), and that these influences can be sensed first and most deeply in secular music of the second half of Henry VIII's reign, rather than in contemporary liturgical music (the study includes a detailed discussion and full transcription of the late Henrician partsong repertory). The close stylistic identity between secular chamber style and that of Edwardian church music is emphasized, in particular their common reliance upon stretto lattices of declamatory imitation. Using Tallis' works as a touchstone, subsequent developments in musical substance and structure are investigated; and the study closes by demonstrating how Tallis' imitative practice changed rapidly and radically during the third quarter of the century, again almost certainly under the influence of foreign music.