The aim of Mudiad Ysgolion Meithrin is to promote the education and development of children under 5 through the medium of Welsh in Nursery Cylchoedd Meithrin (playgroups) and Cylchoedd ‘Ti a Fi' (Parent/Carer and child groups). It aims to enrich and extend the Welsh of children from Welsh speaking homes and to facilitate early bilingualism in children from non-Welsh speaking homes. Consequently, Mudiad Ysgolion Meithrin is looked upon as having a specific role to play in language planning in Wales and is included amongst the main institutions working alongside the Welsh Language Board to achieve a number of aims and objectives in relation to planning the future of the Welsh language. Although it is widely assumed that attending a Cylch Meithrin has a major effect on the Welsh language development of children from both Welsh speaking and non-Welsh speaking homes, up to now no sound evidence exists either for or against this. However, the results of this specific research show, for the first time, that children attending Cylchoedd Mudiad Ysgolion Meithrin for a period of a school year develop significantly across a wide range of Welsh language skills, setting a firm foundation for further linguistic development in Welsh: listening and understanding, speaking and communication, and early literacy skills. The way in which the children's development was assessed in these linguistic areas is discussed and further considerations are raised in order to facilitate the Mudiad to build on the results of the research and continue to increase the number of Welsh speakers.