This thesis consists of a critical edition and translation of a Middle Irish metrical tract known as Mittelirische Verslehren III (hereafter MV III). The tract is an important source of information on rhyming syllabic verse in the Middle Irish period. It consists of two hundred and nine stanzas which illustrate various types of metres, ranging from common metres to more complex metrical forms, many of which are not represented in the standard works of reference. The metres are classified into four sections as gnáthaisti ‘common metres’ (§§2-127), gnátha medónda ‘moderately well-known’ (§§128-47), anaichinti ‘unfamiliar’ (§§148-66) and écoitchenna ‘uncommon’ (§§167-210). The stanzas cited as illustrations cover a wide range of subject matter, including panegyric, satiric, religious and topographical material. At present, the text is available only in a diplomatic edition by Rudolf Thurneysen (1891, 1-182). This is based on two manuscripts: Royal Irish Academy MS 23 P 12 (the Book of Ballymote) and TCD MS H.2.12. He subsequently published (1912, 60- 72) a selection of readings from RIA MS D ii 1 (the Book of Uí Maine). Two further manuscript witnesses of the text were not available to Thurneysen. These are National Library of Scotland Gaelic MS 1 (Advocates 72.1.1) and National Library of Ireland MS G 3, both of which contain important variant readings.