In most of the world, bilingualism is the norm. Even in the US, a primarily monolingual society, there is a growing awareness that knowledge of a second language is essential to our competitiveness in an increasingly interactive world. However, there are a number of issues concerning the cognitive and neural systems that underlie monolingual and bilingual language use that remain unresolved. The research in this third area focuses on one specific but critical component of the mechanisms involved in being bilingual – the cognitive and neural processes involved in acquiring and using a vocabulary in a second language (L2). Using both behavioral and electrophysiological (ERP) techniques, our aim is to plot the cognitive and neural consequences of vocabulary acquisition in a foreign language by examining various stages of L2 language learning in both cross-sectional and longitudinal samples. Behavioral data allows this research to be linked with the large amount of prior behavioral research on these and similar issues; ERP data will allow both quantitative and qualitative changes in the processing of L2 words to be tracked as a function of proficiency with the high degree of temporal accuracy associated with ERPs.