Over the past several decades, linguistic theorizing of tense, aspect, and mood (TAM), along with a strongly growing body of crosslinguistic studies, have revealed complexity in the data that challenges traditional distinctions and treatments of these categories. This book argues that it’s time to revisit our conventional assumptions and reconsider our foundational questions: What exactly is a linguistic category? What kinds of categories to labels such as “subjunctive,” “imperative,” “future,” and “modality” truly refer to? In short, how categorical are categories? Current literature assumes a straightforward link between grammatical category and semantic function, and descriptions of well-studied languages have cultivated a sense of predictability in patterns over time. However, this predictability and stability vanish in the study of lesser-known patterns and languages. The ten provocative essays gathered here present fascinating cutting-edge research demonstrating that the traditional grammatical distinctions are ultimately fluid, and perhaps even illusory. Developing groundbreaking and highly original theories, the contributors in this volume seek to unravel more general, fundamental principles of TAM that can help us better understand the nature of linguistic representations.