  The collision of ultra-relativistic electron beams with intense short laser
pulses makes possible to study QED in the high-intensity regime. Present day
high-intensity lasers mostly operate with short pulse durations of several tens
of femtoseconds, i.e. only a few optical cycles. A profound theoretical
understanding of short pulse effects is important not only for studying
fundamental aspects of high-intensity laser matter interaction, but also for
applications as novel X- and gamma-ray radiation sources. In this article we
give a brief overview of the theory of high-intensity QED with focus on effects
due to the short pulse duration. The non-linear spectral broadening in
non-linear Compton scattering due to the short pulse duration and its
compensation is discussed.
