The book "Computer Aided Systems Theory - EUROCAST 2009" offers a comprehensive analysis and discussion of Computer Aided Systems Theory (CAST), which was first introduced by F. Pichler in the late 1980s. CAST is considered an important complement to the concepts of Computer Aided Design (CAD) and Computer Aided Manufacturing (CAM) and serves as a tool for solving complex problems in systems science. The EUROCAST conferences, held every two years, have established themselves as a significant platform for the exchange of ideas and developments among system theorists, computer scientists, and engineers. This 2009 edition summarizes the results and insights from the latest conferences and provides valuable perspectives on the advancements and challenges in the application of CAST in science and technology.