This book contains 12 papers contributed by leading scholars in the field of language development, studying variants of the languages which originated on the Iberian peninsula. The contributors examine language development in both typically-developing and language-impaired populations who are learning language in diverse learning conditions, including language contact, as well as monolingual and bilingual Spanish, Catalan, Galician and Euskera. This expansion and diversification of the database for studying language development is important because it creates new opportunities for testing theoretical claims. Our contributors reconsider theoretical claims relating to the purported adult-like nature of young children’s grammars. While some conclude, for example, that children in Mexico possess very adult-like semantic-pragmatic competence in the domain of the pragmatic implicatures associated with existential quantifiers, others conclude that, in particular sociolinguistic registers of Chilean Spanish, children are late to develop adult-like competence in plural marking. Taken together, the contents of the volume illustrate how the linguistic diversity found in the distinct learning conditions in which language develops offers a wealth of opportunities to further our understanding of linguistic and non-linguistic cognitive development.