This volume contains the proceedings of TESTCOM/FATES 2008, a joint c- ference of two communities: TESTCOM was the 20th edition of the IFIP TC6/ WG6.1 International Conference on Testing of Communicating Systems and FATESwasthe8theditionoftheInternationalWorkshoponFormalApproaches to Testing of Software. TESTCOM/FATES 2008 was held at the Campus In- vation Center in Tokyo, Japan during June 10-13, 2008. Testing is one of the most important techniques for validating and che- ing the correctness of communication and software systems. Testing, however, is also a laborious and very cost-intensive task during the development process of such systems. TESTCOM is a series of international conferences addressing the problemsoftestingcommunicatingsystems, including communicationprotocols, services, distributed platforms, and middleware. FATES is a series of inter- tional workshopsdiscussing the challengesof using rigorousand formal methods for testing software systems in general. TESTCOM/FATES aims at being a - rum for researchers, developers, and testers to review, discuss, and learn about new approaches, concepts, theories, methodologies, tools, and experiences in the ?eld of testing of communicating systems and software. TESTCOM has a long history. Previously it was called the International Workshop on Protocol Test Systems (IWPTS) and changed its name to the - ternationalWorkshoponTestingofCommunicatingSystem(IWTCS)later.The previous conferences were held in Vancouver, Canada (1988); Berlin, Germany (1989);McLean, USA (1990);Leidschendam, TheNetherlands(1991);Montr eal, Canada (1992); Pau, France (1993); Tokyo, Japan (1994); Evry, France (1995); Darmstadt, Germany(1996);Cheju Island, Korea(1997);Tomsk, Russia(1998); Budapest, Hungary (1999); Ottawa, Canada (2000); Berlin, Germany (2001); Sophia Antipolis, France (2002); Oxford, UK (2004); Montr eal, Canada (2005); New York, USA (2006) and Tallinn, Estonia (200