From a global perspective, bilingual language acquisition can be considered the norm rather than the exception. In bilingual communities around the world, infants exposed from birth to two different languages, or even dialects, succeed in the task of simultaneously learning their two native languages. Infants growing up in this type of environments are exposed to a complex input that contains information relative to two different phonological systems. Early in development bilingual-to-be infants must be able to differentiate the sound patterns of their two languages and start building languagespecific phonetic categories. Research on young bilinguals’ phonetic categorization and perceptual reorganization processes by the end of the first year of life has revealed interesting differences between consonant and vowel categories. Once in the lexical stage, phonetic categories already established will turn into the contrastive categories that form the phonological systems for each of the ambient languages. This is by no means an automatic process. Data from studies with monolingual toddlers participating in word learning tasks have revealed that minimal pair word labels, differing in their initial stop consonant, such as [bih] and [dih], cannot be easily learned at 14 months of age, even though /b/ and /d/ contrastive sounds can be discriminated with no difficulty at the same age (Stager & Werker, 1997). In the case of bilingual toddlers, engaged in the process of establishing two lexicons based on two distinct phonological systems, the situation is even more challenging. There are still relatively few studies specifically focusing on bilinguals’ setting up the phonetic and phonological categories of their native languages (see Werker & Byers-Heinlein, 2008, for a review). Experimental data come mostly from three research groups settled in areas where bilingual populations are available for participation in speech perception studies: J. Werker group at the University of British Columbia in Vancouver (Canada), L. Polka group at McGill University in Montreal (Canada) and the group at the University of Barcelona (Spain) whose main findings will be described in the following sections. Researchers from the above mentioned groups, dealing with bilingual infants and toddlers from various language communities and exposed to different pairs of languages, have all contributed to shed light on the adaptability of the speech processing system to cope with different types of linguistic input. What previous research in bilingual language development had told us, from a general perspective, was that the pattern of acquisition in bilinguals was rather similar to the pattern of acquisition that had been described for monolingual infants: an early language differentiation was suggested as words in both of the ambient languages were present in their initial expressive lexicons (Genesee, Nicoladis & Paradis, 1995; Pearson, Fernandez & Oller, 1995) and they followed the same steps as monolinguals’ in reaching the key milestones in the language acquisition process (Oller, Eilers, Urbano & Cobo-Lewis, 1997). From a phonological acquisition perspective, however, input to bilinguals has specific properties and clearly differs from monolingual input, not only in complexity (two lexicons, two phonologies), but also in quantity and quality of exposure to each language. Moreover, the degree of proximity between the specific lexical, phonological and morpho-syntactical properties of the two ambient languages is also a relevant factor to be taken into consideration. The complex and variable nature of the input to bilingual infants and toddlers can determine minor time-course differences in reaching specific sound discrimination abilities or in stabilizing certain phonetic categories when comparing bilingual and monolingual infants. But, more interestingly, similarities or differences in the phonetic and phonological properties of the two languages in the input can result in differences in perception/discrimination abilities observed in groups of bilinguals from different linguistic environments. Language differentiation processes, the setting up of language-specific phonetic categories, phonological representation of sounds in the lexicon, might differ when comparing bilinguals from different pairs of languages.