Grammaticalization and lexicalization are at the heart of first language acquisition. Understanding how these processes begin and evolve is a major challenge for current theories and has implications for applications in teaching or clinical contexts. This volume examines the relative weight of cognitive and linguistic determinants of acquisition with particular attention to two questions. The first one concerns the origins of grammar and the processes underlying its development. Is grammatical knowledge innate or constructed by the child? Is it modular or does it interact with other capacities? How can we account for continuity and discontinuity in development? What is the role of input? Second, considerable variation is observed in lexical and grammatical development across child languages. Is the process of acquisition similar in all children or do language-specific factors impact its rhythm and course? Do typological factors determine children's reliance on lexical or grammatical means of expression in some domains? Originally published in Language, Interaction and Acquisition - Langage, Interaction et Acquisition 2:1 (2011).