Parts of Speech are a central aspect of linguistic theory and analysis. Though a long-established tradition in Western linguistics and philosophy has assumed the validity of Parts of Speech in the study of language, there are still many questions left unanswered. For example, should Parts of Speech be treated as descriptive tools or are they to be considered universal constructs? Is it possible to come up with cross-linguistically valid formal categories, or are categories of language structure ultimately language-specific? Should they be defined semantically, syntactically, or otherwise? Do non-Indo-European languages reveal novel aspects of categorical assignment? This volume attempts to answer these and other fundamental questions for linguistic theory and its methodology by offering a range of contributions that spans diverse theoretical persuasions and contributes to our understanding of Parts of Speech with analyses of new data sets.