Phi-features, such as person, number, and gender, present a rare opportunity for syntacticians, morphologists and semanticists to collaborate on a research enterprise in which they all have an equal stake and which they all approach with data and insights from their own fields. This volume is the first to attempt to bring together these different strands and styles of research. It presents the core questions, major results, and new directions of this emergent area of linguistic theory and shows how Phi Theory casts light on the nature of interfaces and the structure of the grammar. The book will interest scholars and students of all aspects of linguistic theory at graduate level and above.