This review examines a number of commonly held beliefs about second-language learning. Studies of simultaneous and successive acquisition of a second language in children are reviewed, and the evidence for these beliefs is critically evaluated. It remains to be proven (a) that there is a biologically based critical period for language acquisition, such that young children learn a language more quickly and easily than adults, (b) that second-language learning involves different processes than does first-language learning, (c) that interference is an inevitable consequence of bilingualism, or (d) that bilingualism has negative (or positive) consequences on intelligence, educational attainment, or cognitive functioning.