This study investigated categorisation processes by Welsh/English bilingual and monolingual English children. It focused specifically on the effects of complexity for group membership of a word group, and the amount of input received for both languages, influenced by the home language environment and developmental factors. The study adopted the constructivist model view that bilingual Welsh/ English children exhibit behaviour consistent with processing language using two separate sets of categorisation processes, a semantic system and a conceptual space in which the child develops cognitive abilities to make sense of the world. Children develop both languages in parallel to monolinguals but are delayed in their acquisition of lexical items due to reduced input. The research consisted of four studies, with the main study involving two hundred and twenty eight monolingual and bilingual (Welsh/English) children aged 5-11 completing a picture naming task, either in Welsh or English, with referents presented in four word categories. The word categories were classical, homonyms, radial taxonomic and radial thematic with words always showing a 'wider' range in one language and 'narrower' in the other. The remaining three studies consisted of an analysis of vocabulary tests conducted using the BPVS and Prawf Geirfa Cyrnraeg; the use of a memory task to investigate the research question; and an analysis of errors in selecting related items in the naming task. The lack of taxonomic bias across age groups and category groups was a notable finding as this supported the view that bilingual children rely more on cognitive skills to categorise rather than structured linguistic processes. In addition, there was strong evidence that children had greater difficulty with processing items from the classical word group. This was seen in the poorer performance of bilingual children whose dominant language was English when tested in Welsh. A similar performance was found for the bilingual group whose dominant language was Welsh when tested in English across the three age groups. With children aged 5 to 7, 7 to 9 and 9 to 11 not performing as well as the other three home language groups on items from the classical words group. The results of the study supports the constructivist theory model (Gathercole, 2007) and dual coding processing hypothesis (Paivio & Desrochers, 1980) that bilingual individuals use two separate language systems and a shared imagery system for categorising words. The results also concur with the assumption that input has an effect on improved acquisition and the better processing of complex language structures. This study marks an initial attempt to investigate semantic categorisation by bilingual Welsh/English children and would benefit from further research into the findings utilising supplementary testing methods