  Modern electron linear accelerators are often designed to produce smooth
bunch distributions characterized by their macroscopic ensemble-average
moments. However, an increasing number of accelerator applications call for
finer control over the beam distribution, e.g., by requiring specific shapes
for its projection along one coordinate. Ultimately, the control of the beam
distribution at the single-particle level could enable new opportunities in
accelerator science. This review discusses the recent progress toward
controlling electron beam distributions on the "mesoscopic" scale with an
emphasis on shaping the beam or introducing complex correlations required for
some applications. This review emphasizes experimental and theoretical
developments of electron-bunch shaping methods based on bounded external
electromagnetic fields or via interactions with the self-generated velocity and
radiation fields.
