This thesis is a study of language switching and concept development in Welsh-English bilingual children aged between 3 and 7 years. The main aim is to study these from an interdisciplinary perspective. A second aim is to increase knowledge of Welsh/English bilingualism. The literature review is wide-ranging, discussing both language switching, and bilingual language development. Studies of switching both within sentences and within conversations are considered, as are linguistic, sociological and psychological explanations for the phenomenon. Studies on cognitive differences between bilingual and monolingual children are also discussed. The empirical part of the thesis utilises data drawn from a study carried out at the University College of Wales, and funded by the Welsh Office. The data consists of language switches produced in recordings made of the language of three to six year old Welsh/English bilinguals. These switches are described and analysed in the light of previous literature on the topic, with particular reference to children who switch frequently and to the types of switches produced. Models previously used in the literature for the analysis of such data are discussed and an alternative approach - the levels-of-analysis model - is presented. The second empirical part of the thesis is the analysisof results on a range of concept tests carried out on children aged between five and seven years old. The results obtained by Welsh preferred language children (bilingual) and English preferred language children (monolingual) are compared. It was found that overall there were few differences between these two groups, supporting the original hypothesis that bilingualism does not have a marked effect on cognitive development. Small differences in the direction of a superior performance by English preferred language children are also discussed and possible explanations considered. It is emphasised in this thesis that bilingualism needs to be studied from an interdisciplinary perspective. The levels of analysis model presented here is offered as an approach for doing this.