Energy-
or density-modulated electron beams are widely met in accelerators.
Modulation wavelength may differ from ~nm to more than a cm, depending
on a cause of the modulation. In Free Electron Lasers the electron beam
is pre-modulated by a laser or an RF source forming a microbunched structure
for coherent emission in the following radiator. From other hand, collective
effects, such as Coherent Synchrotron Radiation or Longitudinal Space
Charge, may lead to a parasitic modulation that can substantially degrade
the electron beam quality. In my talk I review some of effects, methods
and applications of modulated bunches, including seeding, collective
effects, amplitude and wavelength control of modulation, diagnostics
and experiments.