This book is the distillation of over 25 years of work by one of the world's most renowned computer scientists. Leslie Lamport, the creator of the LaTeX typesetting system, shows how to write unambiguous specifications of complex computer systems. The first part provides a concise and lucid introduction to specification, explaining how to describe, with mathematical precision, the behavioral properties of a system-what that system is allowed to do. The emphasis here is on safety properties. The second part of Specifying Systems: The TLA+ Language and Tools for Hardware and Software Engineers covers more advanced topics, including liveness and fairness, real time properties, and composition. The book's final two parts provide a complete reference manual for the TLA+ language and tools, as well as a handy mini-manual.