The aim of this research was to explore the impact of Welsh-English bilingualism on the linguistic and cognitive skills of children with moderate learning difficulties (MLD). Despite there being an abundance of literature about the linguistic and cognitive abilities of typically-developing children, little is known regarding bilingual children with moderate learning difficulties. Therefore, the overarching aim of this research is to make a significant contribution to knowledge about the linguistic and cognitive abilities of bilingual children with MLD. Research into the linguistic abilities of typically developing bilingual children often demonstrates that their vocabulary development is slower than that of monolinguals but ‘catches-up’ with increased experience and exposure and literacy research suggests there is no detrimental effect on literacy skills as a result of bilingualism. Research into the executive functioning (or cognitive) abilities of typically developing bilingual children has often demonstrated that bilingual children benefit from executive functioning advantages, not only in linguistic tasks but also in non-linguistic tasks. However, little is known regarding bilingual children with moderate learning difficulties, particularly in the Welsh-English context. Children in this study were either Welsh-English bilinguals from one of three home language backgrounds (L1 Welsh, L1 English or simultaneous bilinguals) attending Welsh-medium education or English monolinguals attending English-medium education in Wales. Children with moderate learning difficulties were compared to typically developing chronologically age matched peers (10-11 year olds) and developmentally age matched peers (7-8 year olds) from each of the home language backgrounds; the monolinguals used as the control group. All of the bilingual children were tested on a range of standardised vocabulary and reading measures, in English and Welsh, and on a number of executive functioning measures (inhibition, switching and attention) that were adapted from the current literature most often discussed in relation to the cognitive effects of bilingualism. The findings of the research were mixed with regard to the impact of bilingualism but indicated that bilingual children with moderate learning difficulties perform at developmentally appropriate levels in both linguistic and cognitive domains, and in some tasks performed better than the developmentally age matched children. Similarly to research into typically developing Welsh-English bilingual children, these findings suggested that home language does impact Welsh and English language development and that bilingual children with MLD do not necessarily ‘catch-up’ with their monolingual peers by age 11. The executive functioning findings indicated no consistent advantage for the bilingual children over the monolingual children. The findings of this research did however indicate that children with moderate learning difficulties possibly responded ‘qualitatively’ differently on the executive functioning tasks which may have implications for education. The findings contribute to the field of bilingualism and moderate learning difficulties. They may also be of use to parents and schools regarding the impact of Welsh-medium education on children who are experiencing difficulties with the curriculum (i.e. children with moderate learning difficulties) and may be of use to educational professionals in planning support for bilinguals with moderate learning difficulties.