This volume contains the proceedings of the 16th IFIP TC6/WG6.1 Inter- tional Conference on Testing of Communicating Systems (TestCom 2004). This conference was held at St Anne's College, Oxford, UK, from March 17 to March 19, 2004. TestCom 2004 was the sixteenth in a series of IFIP-sponsored events that started in 1988. The previous events were held in Vancouver, Canada (1988); B- lin, Germany (1989); McLean, USA (1990); Leidschendam, Netherlands (1991); Montreal, Canada (1992); Pau, France (1993); Tokyo, Japan (1994); Evry, France (1995); Darmstadt, Germany (1996); Cheju Island, Korea (1997); Tomsk, R- sia (1998); Budapest, Hungary (1999); Ottawa, Canada (2000); Berlin, Germany (2002); and Sophia Antipolis, France (2003). TestCom was not held in 2001 since at this point the conference moved from autumn to spring. TestCom 2004 was organized by Brunel University, UK and LSR-IMAG, France and was sponsored by IFIP. Support was also provided by the Engineering and Physical Sciences Research Council (EPSRC). We are grateful to the keynote speaker, Prof. Sir Tony Hoare, FRS, and our invited speakers for agreeing to address TestCom 2004.
