It has been suggested by diagnosticians that alongside the series of Nuclear Fusion special issues dedicated to the physics and technology of heating subsystems for large tokamaks, it would also be worthwhile to publish special issues concentrating on diagnostic systems. Since the field of tokamak diagnostics is very large, it is the intention of the journal to have, every year, one special issue focusing on one specific branch of diagnostics. For this first diagnostics special issue we selected the branch of reflectometry. This method has evolved from a mere auxiliary method to reconstruct density profiles to a sophisticated tool with many applications. These applications range from plasma control to the intricate physics of turbulence. The issue in front of you contains a very interesting mixture of articles on experimental techniques and a wealth of physics observations. They stem from the 7th International Reflectometry Workshop organized by IPP-Garching in May 2005. The workshop gave us a welcome motive to start with this first special issue on diagnostics. I am grateful that we could leave the guest editorship of this issue in the expert hands of Garrard Conway. As an introduction to the subject he has written a summary which appears as the first article of this issue.