Junction at Heol Eglwys, Ystradgynlais
|OS grid reference|
|Sovereign state||United Kingdom|
|Fire||Mid and West Wales|
|Senedd Cymru – Welsh Parliament|
Ystradgynlais (Welsh: [ˌəstradˈɡənlais], English: //) is a town on the River Tawe in southwest Powys, Wales. It is the second-largest town in the principal area and county of Powys. Ystradgynlais is in the historic county of Brecknockshire. The town has a high proportion of Welsh language-speakers. The community includes Cwmtwrch, Abercraf and Cwmgiedd, with a population of 8,092 in the 2011 census; the built-up area population was 10,248. 
The place-name Ystradgynlais, meaning 'vale of the river Cynlais' – Cynlais may be a personal name, or derive from cyn ('chisel') and glais ('stream') – is first recorded in 1372. In the 1600s there were only a couple of houses by the church and a pub (now the rectory). In 1801 there were only 993 residents in the town living in only 196 houses. The first documented written evidence of iron working in the area was at Ynyscedwyn and is of a deed of release dated 1729. By 1750 there were seven furnaces in south Wales, one of which was at Ynyscedwyn.
The first written evidence of coal mining was at Wauclawdd in 1780. Most of the coal dug up was used in the blast furnaces of the ironworks. By 1790, the full extent of the mineral resources in the valley was better known and it was realised that to exploit these to the full, improved transport would be essential.
The greatest increase in the population was between 1821 and 1841. This coincides with the coming of George Crane and the development of the Ynyscedwyn Ironworks. By 1870, the town's industrial development was in decline due to various economic factors, although coal mining carried on in the area with a few light industries.
Ystradgynlais hosted the 1954 National Eisteddfod, an annual Welsh festival of literature, dance, and music. The century-old award-winning Ystradgynlais Public Band competed in the 2005 National Eisteddfod.
Dan yr Ogof caves are a short journey from the town centre, passing Craig-y-Nos Castle and country park. The caves are reputed to have once been the hideout of folk figure Twm Siôn Cati. Henrhyd Falls are also nearby.
Ystradgynlais is also home to the Miners Welfare Hall, known and promoted as 'The Welfare', which contains a cinema. It also has a number of public houses.
In 2016 The Stephen Lewis Tristars Aquathlon in Ystradgynlais won the Welsh triathlon event of the Year 2016
National Cycle Route 43 passes by the southern edge of the town on the line of the former Swansea Vale Railway which linked Swansea via the Neath and Brecon Railway at Coelbren with Brecon. Ystradgynlais railway station was operational from 1869 to 1923.
In 1946, Smiths Industries Ltd, Ingersoll Ltd and Vickers Armstrong founded the Anglo-Celtic Watch Co. Ltd. producing watches on the Ynyscedwyn estate on the outskirts of Ystradgynlais. The factory was officially opened by Hugh Dalton on 15 March 1947. Vickers Armstrong sold their shares to the other two companies in 1948. The company became one of the largest producers of watches in Europe, producing up to 1.25 million watches a year until 1980 when it closed.
The Archbishop of Canterbury from 2002–12, Rowan Williams, grew up in Ystradgynlais and Calvinistic Methodist minister Thomas Levi was also born in the town. Politician Caerwyn Roderick and prison reform campaigner Ben Gunn both grew up in the town.
Ystradgynlais has produced a number of Wales international rugby players, including William Lewis Thomas, Huw David Richards, Anthony Buchanan, Steve Bayliss, and Kevin Hopkins. The town was the birthplace of Welsh international footballer Ronnie Rees (born 4 April 1944), who won 39 caps for Wales.
The Polish painter Josef Herman (1911–2000), spent 11 years living and painting in Ystradgynlais. Artists who grew up in the town include opera singer Adelina Patti, composer Daniel Protheroe, novelist Menna Gallie, and actors Steve Meo and Eve Myles.
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