  Accelerator-based light sources such as storage rings and free-electron
lasers use relativistic electron beams to produce intense radiation over a wide
spectral range for fundamental research in physics, chemistry, materials
science, biology and medicine. More than a dozen such sources operate
worldwide, and new sources are being built to deliver radiation that meets with
the ever increasing sophistication and depth of new research. Even so,
conventional accelerator techniques often cannot keep pace with new demands
and, thus, new approaches continue to emerge. In this article, we review a
variety of recently developed and promising techniques that rely on lasers to
manipulate and rearrange the electron distribution in order to tailor the
properties of the radiation. Basic theories of electron-laser interactions,
techniques to create micro- and nano-structures in electron beams, and
techniques to produce radiation with customizable waveforms are reviewed. We
overview laser-based techniques for the generation of fully coherent x-rays,
mode-locked x-ray pulse trains, light with orbital angular momentum, and
attosecond or even zeptosecond long coherent pulses in free-electron lasers.
Several methods to generate femtosecond pulses in storage rings are also
discussed. Additionally, we describe various schemes designed to enhance the
performance of light sources through precision beam preparation including beam
conditioning, laser heating, emittance exchange, and various laser-based
diagnostics. Together these techniques represent a new emerging concept of
"beam by design" in modern accelerators, which is the primary focus of this
article
