In Wales, bilingual education in Welsh and English has an increasingly high profile and Wales shares international leadership of bilingual education policies and practices alongside other countries where bilingual education flourishes. Ever since the first designated Welsh-medium primary school was opened in 1939, Welsh-medium and bilingual education has spread across Wales. This poses both an opportunity and a challenge to educators, since classes may well contain a wide linguistic variety of native speakers and learners of Welsh. This also gives rise to variations in teaching methodologies and in the allocation of languages across the curriculum, with growing experimentation in the concurrent use of both languages within the same lesson period. New research seeks to develop a profile of language allocation in bilingual schools in Wales, and to construct a typology of bilingual education that is empirical as well as conceptual. It also seeks to critique current typologies of bilingual education. This paper will commence with a brief overview of the development of the Welsh language and its role within the bilingual education system in Wales (including current statistics), before going on to discuss the research work in progress and present some early emerging issues and challenges.