The worlds language system is rapidly changing because of demographic trends, new technologies, and international communication (Graddol, 2004). One of the consequences is that the majority of people now speak more than one language. Such an increasing number of bilingual and multilingual speakers across the world will not only affect communication, it will also affect theoretical perspectives in the cognitive sciences. In line with this trend, the study of cognition and language in bilingual and multilingual speakers has gained prominence in the past decade (for reviews, see Kroll & de Groot, 2005; Li & Green, 2007; van Hell & Tokowicz, in press). Recent research indicates that both languages are active in the bilingual mind in on-line processing and both influence language processing even when the social and linguistic context calls for only one language. The implication is that the bilingual cognitive system is fundamentally permeable across language boundaries. This symposium focuses on new developments in research on the learning, perception, and production of words the basic building blocks of a language in second language learners and proficient bilinguals. How do bilinguals juggle two language systems in one mind? Which cognitive mechanisms do bilinguals use to resolve cross-language competition? How do second language learners integrate new knowledge into an existing cognitive system? In the symposium we present to CogSci 2009, these and related questions will be addressed from different angles, integrating experimental, developmental, modeling, electrophysiological and neuroimaging perspectives.