Old Welsh
The earliest extant sources of a language identifiable as Welsh go back to about the 6th century, and the language of this period is known as Early Welsh. Very little of this language remains. The next main period, somewhat better attested, is Old Welsh (Hen Gymraeg) (9th to 11th centuries); poetry from both Wales and Scotland has been preserved in this form of the language. As Germanic and Gaelic colonisation of Great Britain proceeded, the Brythonic speakers in Wales were split off from those in northern England, speaking Cumbrian, and those in the south-west, speaking what would become Cornish, and so the languages diverged. Both "Canu Aneirin" and "Canu Taliesin" were in this era.

Middle Welsh
Middle Welsh (or Cymraeg Canol) is the label attached to the Welsh of the 12th to 14th centuries, of which much more remains than for any earlier period. This is the language of nearly all surviving early manuscripts of the Mabinogion, although the tales themselves are certainly much older. It is also the language of the existing Welsh law manuscripts. Middle Welsh is reasonably intelligible, albeit with some work, to a modern-day Welsh speaker.

Modern Welsh
Modern Welsh can be divided into two periods. The first, Early Modern Welsh ran from the 14th century to roughly the end of the 16th century and was the language used by Dafydd ap Gwilym.

Late Modern Welsh
Late Modern Welsh began with the publication of William Morgan's translation of the Bible in 1588. Like its English counterpart, the King James Version, this proved to have a strong stabilising effect on the language, and indeed the language today still bears the same Late Modern label as Morgan's language. Of course, many minor changes have occurred since then.

19th century
The language enjoyed a further boost in the 19th century, with the publication of some of the first complete and concise Welsh dictionaries. Early work by Welsh lexicographic pioneers such as Daniel Silvan Evans ensured that the language was documented as accurately as possible, and modern dictionaries such as the Geiriadur Prifysgol Cymru (the University of Wales Dictionary), are direct descendants of these dictionaries.

However, the influx of English workers during the Industrial Revolution in Wales from about 1800 led to a substantial dilution of the Welsh-speaking population of Wales. English migrants seldom learnt Welsh and their Welsh colleagues tended to speak English in mixed Welsh–English contexts, and bilingualism became almost universal. The legal status of Welsh was inferior to that of English, and so English gradually came to prevail, except in the most rural areas, particularly in north west and mid Wales. An important exception, however, was in the non-conformist churches, which were strongly associated with the Welsh language.

20th and 21st centuries
By the twentieth century, the numbers of Welsh speakers were shrinking at a rate which suggested that it would be extinct within a few generations. The 10-yearly census first started to ask language questions in 1891, by which time 54% of the population still spoke Welsh. The percentage fell with every subsequent census, until reaching an all-time low in 1981 (19%). In 1991 the position was stable (19% as in 1981) and in the most recent census, 2001, it has risen to 21% able to speak Welsh. The 2001 census also recorded that 20% could read Welsh, 18% could write Welsh, and 24% could understand Welsh. Furthermore, the highest proportion of Welsh speakers was among young people, which bodes well for the future of Welsh. In 2001, 39% of children aged 10 to 15 were able to speak, read and write Welsh (many of them having learned it in school), compared with 25% of 16 to 19 year olds. However, the percentage of Welsh speakers in areas where Welsh is spoken by the majority is still in decline.

It seems that the rise of Welsh nationalism rallied supporters of the language, and the establishment of Welsh television and radio found a mass audience which was encouraged in the retention of its Welsh. Perhaps most important of all, at the end of the twentieth century it became compulsory for all school children to learn Welsh up to age 16, and this both reinforced the language in Welsh-speaking areas and reintroduced at least an elementary knowledge of it in areas which had become more or less wholly Anglophone. The decline in the percentage of people in Wales who can speak Welsh has now been halted, and there are even signs of a modest recovery. However, although Welsh is the daily language in many parts of Wales, English is almost universally understood.