The aim of the present thesis was to investigate the relationship of language and though by examining the process of categorisation and word meaning retrieval in bilingual and monolingual samples. The study of categories has been dominated by the notion that categories are conceptually independent of language. In the present thesis, we adopt the view that, given that the development of conceptual knowledge is parallel to linguistic development, categories are shaped by language in the process. In the present research, different types of categories were used. These category types ranged from those that were more conceptually homogeneous (less linguistically dependent) and those that were more linguistically dependent. Four studies are reviewed in the present thesis. In the first study, participants were required to judge which pictures of objects corresponded to a certain category in two conditions, a "linguistic" condition where the name of the category was provided and a "non linguistic" condition where no category name was available. In the second study, participants were tested on a conscious within category semantic priming study using a lexical decision paradigm. The third study involved ERP measures of a delayed semantic decision task, again using different category types. Finally, the last study used fMRI to examine the neural correlates of different categories in a masked semantic priming task. The results of these four studies provide evidence for the differentiation of the category types, and additionally, support the notion of cross-linguistic influence in category structure and lexical and semantic retrieval.