There is no formal evidence for a cult of Illtyd surviving from before the 11th century. However, in Celtic countries it is the names of places that tell us most about the existence and veneration of the saints during the oldest times. The town of Llanilltud Fawr (Llantwit Major in English) where Illtud's college is located is of course named for him (Welsh: llan church enclosure + Illtud + mawr great. Literally, Illtud's great church), and was the chief centre of the cult of St Illtud. In Glamorgan many churches are dedicated to him, first and foremost St Illtyd's Church, Llantwit Major, which stands on what is believed to have been the site of the monastery. Many other places are dedicated to him is because they belonged to the Llantwit monastery. Near to Llantwit itself are the villages of Llantrithyd, Llantwit Fardre, and Llantrisant and at Newcastle and Bridgend churches are dedicated to St Illtud. In Brecknockshire, the church at Llanhamlach east of Brecon is dedicated to him, and lies south of a Megalithic grave called Ty Illtud, which was a site of mediaeval pilgrimage, the inside walls of the grave bearing incised crosses. The grave is thought to have been a retreat of the saint, as was a similar megalithic monument Roc'h Ildut near Coadut (Coat Ildut/Coed Illtud/ Illtud's Wood) in Brittany, demolished in the 19th century. Llantrisant's three saints were Illtud, Gwynno and Tyfodwg. In Merthyr Tudful there are holy wells of Gwynno and Illtud. West of Brecon, the church of Llanilltud is on a mountain known as Mynydd Illtud. Dedications in and around Gower include Ilston, formerly Llanilltud Gwyr, Oxwich, a holy well of S. Iltut in Llanrhidian, Llanilltud Fach, or Llantwit-juxta-Neath and Pen-bre. A 13th century church on Caldey Island, Pembrokeshire, is dedicated to the saint. In North Wales, there is a Llanelltyd near Dolgellau. In Brittany, there are approximately 24, if other place-names like Aberildut are included; largely confined to the ancient dioceses of Leon, Treguier and Vannes, with small outliers in the region of Saint Malo, originally in the diocese of his pupil Samson.