This thesis contains three large studies into language switching. Three unresolved issues remained after reviewing the most representative accounts of bilingual language comprehension and production: 1. Are linguistic and non-linguistic switching controlled by the same mechanisms? 2. What is the effect of language dominance on switch costs? 3. How does the degree of similarity between words in different languages influence language switch costs? The first, behavioral, experiment addressed how bilinguals detect switches in sentences. The second, ERPs, experiment was aimed at elucidating the temporal pattern of the processing engendered when bilinguals read a sentence containing font or language switches. The third, fMRI, experiment was targeted at unraveling the neural correlates of language switching in language production. All evidence provided by the three experiments suggests that language switching effects arise from both control processes “outside” the lexicon and from the automatic modulation of word activation levels “inside” the lexicon.