Satisfiability (SAT) solvers have become complex tools, which raises the question of whether we can trust their results. This question is particularly important when the solvers are used to determine the correctness of hardware and software and when they are used to produce mathematical results. To deal with this issue, solvers can provide proofs of unsatisfiability to certify the correctness of their answers. This chapter presents the history and state-of-the-art of producing and validating proofs of unsatisfiability. The chapter covers the most popular proof formats with and without hints to speed up certification. Hints in proofs make validation easy, which resulted in several efficient formally-verified checkers. Various proof systems are discussed, ranging from resolution to the recent propagation redundancy system. The chapter also describes techniques to compress and optimize proofs.