The ability to know and to use two or more languages fluently and appropriately reflects the remarkable functioning of a cognitive system that maintains separa tion among the languages being used while at the same time allows free and easy interchange among them. The perplexing complexity of bilingual cognitive func tioning is belied by the apparent ease of performance that is manifested by the proficient bilingual. Much basic research still remains to be done before we understand how bilingual cognition operates but there are signs that more and more researchers are being attracted to this area. In contrast, within the related area of how children become bilingual and whether bilingualism has any particu lar effect on cognitive development, there has been much research and discussion though not complete agreement on the findings. In this chapter we examine this research from three perspectives: first, a historical examination of the issues, highlighting the dramatic changes that have occurred in the past 20 years; sec ond, an examination of methodological issues involved and how they relate to the claims that have been made; third, what role theory has played in guiding research and what role it may play in the future.