This profound exploration of one of the core notions of philosophy—the concept of existence itself—reviews, then counters (via Meinongian theory), the mainstream philosophical view running from Hume to Frege, Russell, and Quine, summarized thus by Kant: "Existence is not a predicate." The initial section of the book presents a comprehensive introduction to, and critical evaluation of, this mainstream view. The author moves on to provide the first systematic survey of all the main Meinongian theories of existence, which, by contrast, reckon existence to be a real, full-fledged property of objects that some things possess, and others lack. As an influential addition to the research literature, the third part develops the most up-to-date neo-Meinongian theory called Modal Meinongianism, applies it to specific fields such as the ontology of fictional objects, and discusses its open problem.