This book presents the essence of his various works—the quest for effective abstractions—both in his own words as well as chapters written by leading experts in the field, including many of his research collaborators. In addition, this volume contains biographical material, his Turing award lecture, the transcript of an interview and some of his seminal papers. Hoare’s foundational paper “An Axiomatic Basis for Computer Programming”, presented his approach, commonly known as Hoare Logic, for proving the correctness of programs by using logical assertions. Hoare Logic and subsequent developments have formed the basis of a wide variety of software verification efforts. Hoare was instrumental in proposing the Verified Software Initiative, a cooperative international project directed at the scientific challenges of large-scale software verification, encompassing theories, tools and experiments. Tony Hoare’s contributions to the theory and practice of concurrent software systems are equally impressive. The process algebra called Communicating Sequential Processes (CSP) has been one of the fundamental paradigms, both as a mathematical theory to reason about concurrent computation as well as the basis for the programming language occam. CSP served as a framework for exploring several ideas in denotational semantics such as powerdomains, as well as notions of abstraction and refinement. It is the basis for a series of industrial-strength tools which have been employed in a wide range of applications. This book also presents Hoare’s work in the last few decades. These works include a rigorous approach to specifications in software engineering practice, including procedural and data abstractions, data refinement, and a modular theory of designs. More recently, he has worked with collaborators to develop Unifying Theories of Programming (UTP). Their goal is to identify the common algebraic theories that lie at the core of sequential, concurrent, reactive and cyber-physical computations.