The development
and reliable operation of high brightness electron sources are key components
for next generation short wavelength light sources and a future linear
collider. This contribution will concentrate on photo injector developments
for different projects which require individual solutions and face specific
problems and challenges. A selective overview on developments and experimental
results will be presented showing that for a large parameter range dedicated
photo injectors are able to full-fill the specific stringent requirements
on beam quality and other beam parameters like e.g. time structure of
the beam.