An observation in recent psycholinguistic research is that the use of a second language (L2) has profound consequences for cognitive processes that extend beyond language itself. Contrary to the notion that the task for the L2 learner is simply to acquire a level of skill that closely resembles that of the native speaker, current research suggests that the language system itself is fundamentally open, with interactions that reshape language use and that carry domain-general cognitive consequences for the ability to resolve competing alternatives. Even proficient bilinguals appear to be unable to selectively switch off the language not in use when they hear, read, or speak one language alone. These interactions can also be seen in bilinguals whose two languages differ markedly in form, as is the case for deaf readers who use a sign language to communicate but acquire literacy in a written language.This paper reviews the evidence concerning the way in which the bilingual’s two languages interact and the consequences for cross-language competition that may result. It compares the results of studies with unimodal bilinguals (two spoken languages) with the emerging literature on bimodal bilinguals (one signed and one spoken or written language) to evaluate the scope of cross-language interactions and the constraints that distinct language form may play in this process.