This volume contains the proceedings of FORTE 2003, the 23rd IFIP TC 6/ WG 6.1 International Conference on Formal Techniques for Networked and D- tributed Systems, held in Berlin, Germany, September 29–October 2, 2003. FORTE denotes a series of international working conferences on formal descr- tion techniques (FDTs) applied to computer networks and distributed systems. The conference series started in 1981 under the name PSTV. In 1988 a s- ond series under the name FORTE was set up. Both series were united to FORTE/PSTV in 1996. Two years ago the conference name was changed to its current form. The last ?ve meetings of this long conference series were held in Paris, France (1998), Beijing, China (1999), Pisa, Italy (2000), Cheju Island, Korea (2001), and Houston, USA (2002). The 23rd FORTE conference was especially dedicated to the application of formal description techniques to practice, especially in the Internet and c- munication domain. The scope of the papers presented at FORTE 2003 covered the application of formal techniques, timed automata, FDT-based design, v- i?cation and testing of communication systems and distributed systems, and the veri?cation of security protocols. In addition, work-in-progress papers were presented which have been published in a separate volume.