Why is learning the grammar of a second language difficult? The present volume brings together recent insights from leading researchers in the U.S. and Europe to identify the multiple factors that challenge learners in attaining full proficiency in a second language. DeKeyser's introductory review synthesizes recent research and theories, illuminating the concept of difficulty in second language acquisition, emphasizing the relations between meaning and forms of a first and second language and their interactions with learners' personal characteristics. A meta-analysis of past research by Goldschneider and DeKeyser highlights the importance of transparency in form-meaning relations as well as learners' aptitudes and previous language learning experiences. Kempe and Brooks show how diminutives facilitate gender acquisition in Russian. Carroll demonstrates how learners make semantic distinctions in acquiring gender distinctions in French. Williams and Lovatt evaluate the role of phonological short-term memory in the acquisition of gender in a semi-artificial linguistic system.