This book provides a critical review of recent theories of semantics-syntax correspondences and makes new proposals for constraints on semantic structure relevant to syntax. Data from several languages are presented which suggest that semantic structure in root morphemes is subject to parametric variation which has effects across a variety of verb classes, including locatives, unaccusatives, and psych verbs. The implications for first and second language acquisition are discussed. In particular, it is suggested that different parametric settings may lead to a learnability problem if adult learners do not retain access to sensitivity to underlying semantic organization and morphological differences between languages provided by Universal Grammar. An experiment with Chinese-speaking learners of English is presented which shows that learners initially transfer L1 semantic organization to the L2, but are able to retreat from overgeneralizations and achieve native-like grammars in this area.Suggestions for further research in this rapidly developing area of theory and acquisition research are also made.