This book offers a new contribution to the debate concerning the acquisition of the syntax-discourse interface. It provides evidence that children acquiring European Portuguese have a very early ability to spontaneously produce VP ellipsis as answers to yes-no questions. It is also argued that the distribution of VP ellipsis in European Portuguese (including its co-existence with Null Complement Anaphora) supports the hypothesis that the identification condition on ellipsis is derivable from some innate knowledge of the syntax-discourse interface. Answers to yes-no questions also provide evidence concerning children's interpretation of questions containing a cleft or the operator so `only'. The analysis of spontaneous production is complemented by a comprehension experiment, showing that children have two problems in the interpretation of these questions: (i) they do not understand that the cleft and so introduce a presupposition and (ii) they start with a default focus assignment strategy and may not access other focus interpretations.