Book excerpt: This volume includes a selection of papers that address a wide range of acquisition phenomena from different Romance languages and all share a common theoretical approach based on the Principles and Parameters theory. They favour, discuss and sometimes challenge traditional explanations of first and second language acquisition in terms of maturation of general principles universal to all languages. They all depart from the view that language acquisition can be explained in terms of learning language specific rules, constraints or structures. The different parts into which this volume is organized reflect different approaches that current research has offered, which deal with issues of development of reflexive pronouns, determiners, clitics, verbs, auxiliaries, Inflection, wh-movement, rssumptive pronouns, topic and focus, mood, the syntax/discourse interface, topic and focus, and null arguments.