Book excerpt: The main focus of generative language development research in recent decades has been the logical problem of language acquisition - how learners go beyond the input to acquire complex linguistic knowledge. This collection deals with the complementary issue of the developmental problem of language acquisition: How do learners move from one developmental stage to another and how and why do grammars develop in a certain fashion? Building on considerable previous research, the authors address both general and specific issues related to paths of development. These issues are tackled through considering studies of L1 and L2 children and L2 adults learning a range of languages including Dutch, English, French, German, Greek and Japanese.