The 3rd IEEE Theoretical Aspects of Software Engineering Conference (TASE 2009), sponsored by IEEE CS and IFIP, will be held in Tianjin, China in July, 2009. Large scale software systems and the Internet are of growing concern to academia and industry. This poses new challenges to the various aspects of software engineering, for instance, the reliability of software development, web-oriented software architecture and aspect and object-orientation techniques. As a result, new concepts and methodologies are required to enhance the development of software engineering from theoretical aspects. TASE 2009 is a forum for researchers from academia, industry and government to present ideas, results, and ongoing research on theoretical advances in software engineering. TASE 2009 is the third in a series of conference, sponsored by IEEE CS and IFIP. The first TASE conference was held in Shanghai, China, in June 2007. The second TASE conference was held in Nanjing, China, in June 2008.