"Software Engineering with OBJ: Algebraic Specification in Action" provides a comprehensive introduction to the OBJ system, which is considered one of the most widely used algebraic specification systems. This textbook covers the formal specification language OBJ, which helps make specifications and designs more precise and readable. Through the capability of execution, whether by symbolic execution with term rewriting or through theorem proving, OBJ distinguishes itself from other specification languages. A central concern in software development is the validation of specifications, and OBJ offers tools for executing test cases as well as proving properties. The book also includes case studies from leading practitioners that demonstrate the versatile applicability of OBJ in areas such as computer graphics, hardware design, and parallel computation. These case studies illustrate that OBJ is not only easy to understand and implement but also supports formal reasoning in a clear and effective manner.