  Short wavelength Free-Electron Lasers (FELs) are the newest light sources
available to scientists to probe a wide range of phenomena, with chemical,
physical and biological applications, using soft and hard X-rays. These sources
include the currently most powerful light sources in the world (hard X-ray
sources) and are characterised by extremely high powers and high transverse
coherence, but the first FELs had reduced longitudinal coherence. Now it is
possible to achieve good longitudinal coherence (narrow bandwidth in the
frequency domain) and here we discuss and illustrate a range of experiments
utilising this property, and their underlying physics. The primary applications
are those which require high resolution (for example resonant experiments), or
temporal coherence (for example coherent control experiments). The currently
available light sources extend the vast range of laboratory laser techniques to
short wavelengths.
