The cusps have traditionally been described as narrow funnel-shaped regions that provide a focus of the Chapman–Ferraro currents that flow on the magnetopause, a boundary between the cavity dominated by the geomagnetic field (i.e., the magnetosphere) and the external region of the interplanetary medium. From low-altitude satellite and ground-based measurements the cusps appear to behave in much the manner predicted for the responses of the narrow funnel-shaped structures to changes in the upstream interplanetary medium. Measurements at higher altitudes have been reported by past and recent missions such as the Russian Interball satellites, the US/NASA Hawkeye, Polar, and IMAGE satellites, and the joint European Space Agency/NASA Cluster suite of four satellites, all in mid- to high-altitude polar orbits. From these measurements, it has become clear that the cusps are no longer confined to narrow regions near local noon but appear to encompass a large portion of the dayside high-latitude magnetosphere. An unexpected result is that the cusps appear to be a major source region of energetic charged particles for the magnetosphere.