Only human beings have language in the sense of a system of signs that allows them to express communicative intentions in an unlimited variation. Both biological and environmental factors play a role in acquiring a language. However, the exact balance of nature and nurture, especially which biological features are relevant for language acquisition, has been hotly debated. In studying language acquisition cross-linguistically, we learn about the range of capacities required for language learning. Children can learn any language even though languages vary extremely in their structures. Every normally developing child will learn the language or, as is the case in most cultures of the world, the languages of their community in the fi rst years of their lives, including children brought up with a sign language.* How do children learn such complex and variable systems? In all of the languages studied, children follow approximately the same timetable in the major landmarks of development such as babbling, fi rst words, and fi rst complex utterances. But how do children learn the structures of their languages and is there uniformity in the strategies they use even if they are learning languages with completely diff erent structures? To assess the task of the language learner and discover the strategies children employ, it is important to study the acquisition of as wide a range of languages as possible and to conduct systematic cross-linguistic comparisons of acquisition strategies.