This is the first book entirely and exclusively devoted to the grammar of the two copular verbs ser and estar, certainly one of the most intriguing features of Spanish grammar. Although the topic has long attracted the interest of scholars, it had never given rise to a collection of papers that covers both theoretical issues in syntax and semantics and topics in the acquisition domain. This volume provides a comprehensive overview of the central research questions concerning the ser / estar alternation: the syntactic or semantic nature of the distinction, its link with aspect and with the Individual-Level / Stage-Level distinction, and its connection with interface phenomena. This book is a must-read for anyone interested in Hispanic linguistics, but can be equally attractive for researchers working on Romance linguistics, theoretical linguistics (syntax, semantics, pragmatics), acquisition theory, and historical linguistics.