The extension UMLsec of the Unified Modeling Language for secure systems development is presented in this text. The book is written in a way which keeps the first part accessible to anyone with a basic background on object-oriented systems. The second part covers the mathematical tools needed to use the UMLsec approach to verify UML specifications against security requirements. It can also be used as part of a general course on applying UML or on computer security. A practically relevant example is used throughout the book to demonstrate the presented methods.