This special issue of Bilingualism: Language and Cognition is devoted to syntactic aspects of bilingual acquisition. For the purposes of this issue, bilingual acquisition is defined as the acquisition of two languages during the period of primary language development, extending from birth onward. Bilingual acquisition can entail the acquisition of more than two languages (see Cenoz and Jessner, 2000) as well as the acquisition of a spoken and signed language (e.g., Richmond-Welty and Siple, 1999) or of two spoken languages; only studies of the simultaneous acquisition of two spoken languages are reported in this volume. An ideal definition of bilingual acquisition would include not only reference to the age of first exposure to two languages, but also reference to the regularity and extent of exposure to each language. While such stipulations are not necessary in defining the context for monolingual acquisition since virtually all children receive sufficient language exposure to fully acquire one language, they are important considerations in cases of bilingual acquisition since some children have insufficient exposure to two languages, either in terms of amount or continuity, to attain full bilingual proficiency.