The correct development of security-critical computer systems is as vital as it is difficult. This book presents the extension UMLsec of the Unified Modeling Language for secure systems development. The book is written in a way which keeps the first part (ch 1-5), describing UMLsec and ist use, accessible to anyone with an interest in computer security and a basic background on object-oriented systems. It can also be used as part of a general course on applying UML or on computer security. The second part (ch 6-9) is about the mathematical tools needed to use the UMLsec approach to verify UML specifications against security requirements and assumes some familarity with precise methods of system development. A practically relevant example is used throughout the book to demonstrate the presented methods