Thomas Tomkins (1572-1656), organist of Worcester Cathedral and of the Chapel Royal, was one of the most significant English composers in the first half of the seventeenth century. His sacred and secular vocal music has become widely known through modern editions, but although his solo keyboard music has been available in print since 1955 it has received little critical attention and is seldom played. His output for string consort in three to six polyphonic parts has fared even worse: although playing parts of some of the consort pieces have appeared, these are not readily available, and are normally based on only one contemporary manuscript source. At present, therefore, our picture of Tomkins's overall achievements as a composer is incomplete, and therefore distorted, owing to the lack of a detailed consideration of his instrumental output. A critical study of the keyboard music, a complete edition of the consort music, collated from all existing contemporary manuscript sources, with a paleographical assessment of these sources, and an attempt to place this newly edited material in context, define, collectively, the scope of this thesis. Although many of Tomkins's keyboard works are dated in the composer's manuscript, a strictly chronological assessment of these has not been adopted since this reveals far less of the composer t s diversity than successive treatment by genre (preludes, plainsong settings, fantasias, grounds, pavans, variations and miscellanea). Several general topics which do not fit comfortably into these specific categories are dealt with in Appendices following the critical and paleographical chapters. The transcriptions, with accompanying editorial notes and commentaries, are presented in a separate volume.