The rapid development of new technologies over the past decade has greatly expanded the capabilities of relativistic electron beams. While electron beams constitute one of the oldest particle beams available to physicists, the strengths of such beams were limited to a few amperes up to the early 1960's. Since then, however, a new type of accelerator has been developed that permits electron‐beam bursts of many kiloamperes—and even megamperes—to be accelerated to energies of several megavolts. The most recent facility of this type at Cornell University, shown on the cover of this issue ofPHYSICS TODAY, is capable of accelerating a 15‐kA beam of electrons to 5 MeV. Figure 1 shows Aurora, the largest facility of this type presently operating, which is able to accelerate 160‐nanosecond pulses of electron beams of up to 1.6 megamperes to energies as high as 12 MeV.