The notion of negation is one of the central logical notions. It has been studied since antiquity and has been subjected to thorough investigations in the development of philosophical logic, linguistics, artificial intelligence and logic programming. The properties of negation -- in combination with those of other logical operations and structural features of the deducibility relation -- serve as gateways among logical systems. Therefore negation plays an important role in selecting logical systems for particular applications. At the moment negation is a 'hot topic', and there is an urgent need for a comprehensive account of this logical key concept. We have therefore asked leading scholars in various branches of logic to contribute to a volume on What is Negation? The result is the present neatly focused collection of research papers bringing together different approaches to a general characterization of kinds of negation and classifications thereof. Audience: Scholars and graduate students in the fields of philosophy, logic mathematics, computer science and linguistics.