The book "What is Negation?" offers a comprehensive examination of the central logical concept of negation, which has been intensively studied since ancient times. It highlights the role of negation in various disciplines such as philosophical logic, linguistics, artificial intelligence, and logic programming. The authors, leading scholars in their fields, present different approaches to the characterization and classification of negation in this anthology. The structure of the book is divided into four thematic parts that address models, relevance, and impossibility. The contributions provide both semantic and proof-theoretical perspectives on negation and discuss its significance for the selection of logical systems in specific applications. This collection is particularly relevant in a context where negation is considered a current and significant topic.