This collection of papers, published in honour of Hector J. Levesque on theoccasion of his 60th birthday, addresses a number of core areas in the field ofknowledge representation and reasoning. In a broad sense, the book is aboutknowledge and belief, tractable reasoning, and reasoning about action andchange. More specifically, the book contains contributions to DescriptionLogics, the expressiveness of knowledge representation languages, limited formsof inference, satisfiablity (SAT), the logical foundations of BDI architectures, only-knowing,belief revision, planning, causation, the situation calculus, the actionlanguage Golog, and cognitive robotics.