Z3 is an efficient Satisfiability Modulo Theories (SMT) solver from Microsoft Research. Z3 is a solver for symbolic logic, a foundation for many software engineering tools. SMT solvers rely on a tight integration of specialized engines of proof. Each engine owns a piece of the global puzzle and implements specialized algorithms. For example, Z3’s engine for arithmetic integrates Simplex, cuts and polynomial reasoning, while an engine for strings are regular expressions integrate methods for symbolic derivatives of regular languages. A theme shared among many of the algorithms is how they exploit a duality between finding satisfying solutions and finding refutation proofs. The solver also integrates engines for global and local inferences and global propagation. Z3 is used in a wide range of software engineering applications, ranging from program verification, compiler validation, testing, fuzzing using dynamic symbolic execution, model-based software development, network verification, and optimization.