This book constitutes the refereed proceedings of the 21st International Conference on Theory and Applications of Satisfiability Testing, SAT 2018, held in Oxford, UK, in July 2018. The 20 revised full papers, 4 short papers, and 2 tool papers were carefully reviewed and selected from 58 submissions. The papers address different aspects of SAT interpreted in a broad sense, including theoretical advances (such as exact algorithms, proof complexity, and other complexity issues), practical search algorithms, knowledge compilation, implementation-level details of SAT solvers and SAT-based systems, problem encodings and reformulations, applications as well as case studies and reports on findings based on rigorous experimentation. They are organized in the following topical sections: maximum satisfiability; conflict driven clause learning; model counting; quantified Boolean formulae; theory; minimally unsatisfiable sets; satisfiability modulo theories; and tools and applications.