  Critical progress in our understanding of high energy emission from AGN has
been determined in the last 10 years by X-ray monitoring campaigns with many
space missions, notably ROSAT, ASCA, RXTE, BeppoSAX, and XMM, often in
conjunction with observations at other frequencies. The emphasis of the present
review is on recent findings about X-ray variability of blazars. Among AGN,
these exhibit the largest amplitude variations of the X-ray emission, often
well correlated with variations at higher energies (GeV and TeV radiation). The
accurate sampling of the X-ray spectra over more than three decades in energy,
made possible by the wide energy range of BeppoSAX, has also shown strong
spectral variability in blazar active states, suggesting extreme electron
energies and leading to the identification of a class of `extreme synchrotron'
sources.
