Ieuan Gwyllt began to compose music when he was quite young; a hymn-tune by him, ‘Hafilah,’ was published in Nov. 1839 in Yr Athraw. In 1852 he published Blodau Cerdd which contained lessons in music together with hymn-tunes which, like ‘Hafilah,’ were imitations of the poorer kind of English hymn-tunes. It was after he went to Liverpool (1852) that he came to recognize what was characteristic of true ecclesiastical hymn-tunes, and it was then that he began to start on his real life-work — the collection and selection of the best tunes for use by his countrymen. After labouring for six years he was able to produce (April 1859) Llyfr Tonau Cynulleidfaol, with the publication of which there began a new era of Welsh congregational singing. To the original work he added an atodiad (supplement), and, in 1870, an ychwanegiad (appendix). He arranged and harmonized a large number of hymn-tunes and psalms and himself composed some two dozen including the very famous tune ‘Moab.’ At this time he was travelling much throughout Wales to lecture on congregational music. In 1859 he and Thomas Levi (q.v.), Aberystwyth, issued the first number of Telyn y Plant, the name of which was changed in 1861 to Trysorfa y Plant; Ieuan Gwyllt was responsible for the hymn-tunes. In 1859 he was asked to become minister of Pant-tywyll Calvinistic Methodist church, Merthyr Tydfil; he was ordained on 7 Aug. 1861 at the Newcastle Emlyn Association. In March 1861 he issued the first number of Y Cerddor Cymreig, a periodical which he continued to edit and publish on his own responsibility for four years, i.e. until Hughes and Son, Wrexham, took it over (1865); Ieuan Gwyllt continued as editor until 1873. He founded the Gwent and Morgannwg musical festival in 1854, Gŵyl Eryri in 1866, and Gŵyl Ardudwy in 1868. He began to study the Tonic Sol-fa system in 1863, producing a sol-fa edition of his Llyfr Tonau Cynulleidfaol the next year. He founded Cerddor y Tonic Solffa in 1869 and edited it until 1874. In 1865 he became minister of Capel Coch Calvinistic Methodist church, Llanberis, Caerns., where he remained until he retired to Y Fron, Llanfaglan, near Caernarvon, in 1869. He was secretary of the committee which prepared Llyfr Emynau y Methodistiaid Calfinaidd, 1869. He edited Y Goleuad, from July 1871 until Oct. 1872. In 1874 he issued Sŵn y Jiwbili, an arrangement in Welsh of Sankey and Moody hymns and tunes. Throughout the years he was well known as music adjudicator and as conductor of singing-festivals; he was also a frequent contributor to Y Traethodydd and Yr Oenig. He d. 14 May 1877 and was buried in Caeathro cemetery, near Caernarvon.