Book excerpt: This handbook explores multiple facets of the study of word classes, also known as parts of speech or lexical categories. These categories are of fundamental importance to linguistic theory and description, both formal and functional, and for both language-internal analyses and cross-linguistic comparison. The volume consists of five parts that investigate word classes from different angles. Chapters in the first part address a range of fundamental issues including diversity and unity in word classes around the world, categorization at different levels of structure, the distinction between lexical and functional words, and hybrid categories. Part II examines the treatment of word classes across a wide range of contemporary linguistic theories, such as Cognitive Grammar, Minimalist Syntax, and Lexical Functional Grammar, while the focus of Part III is on individual word classes, from major categories such as verb and noun to minor ones such as adpositions and ideophones. Part IV provides a number of cross-linguistic case studies, exploring word classes in families including Afroasiatic, Sinitic, Mayan, Austronesian, and in sign languages. Chapters in the final part of the book discuss word classes from the perspective of various sub-disciplines of linguistics, ranging from first and second language acquisition to computational and corpus linguistics. Together, the contributions showcase the importance of word classes for the whole discipline of linguistics, while also highlighting the many ongoing debates in the areas and outlining fruitful avenues for future research.