The problem of lexical categories and root class determination is fundamental in linguistic description and theory. Research on this topic has been particularly stimulated by studies of Amerindian languages. The essays in this collection, written by specialists in languages from South, Middle and North America, provide new insights into processes, levels, functions, and the aquisition of lexical categories, from various recent theoretical perspectives. The volume also addresses recent debates about root indeterminacy. Focusing on morphosyntax, phonology, and semantics, the contributions offer invaluable material for typological generalizations and for comprehension of the nature of the mental lexicon.