This book presents a collection of chapters on the nature, flexibility and acquisition of lexical categories. These long-debated issues are looked at anew by exploring the hypothesis of lexical polycategoriality –according to which lexical forms are not fully, or univocally, specified for lexical category– in a wide number of unrelated languages, and within different theoretical and methodological perspectives. Twenty languages are thoroughly analyzed. Apart from French, Arabic and Hebrew, the volume includes mostly understudied languages, spoken in New Guinea, Australia, New Caledonia, Amazonia, Meso- and North America. Resulting from a long-standing collaboration between leading international experts, this book brings under one cover new data analyses and results on word categories from the linguistic and acquisitional point of view.