  Aperture Arrays have played a major role in radio astronomy since the field
emerged from the results of long-distance communication tests performed by Karl
Jansky in the early 1930's. The roots of this technology extend back beyond
Marconi, although the first electronically scanned instrument only appeared in
the run-up to World War II. After the war, phased arrays had a major impact in
many walks of life, including astronomy and astrophysics. Major progress was
made in understanding the nature of the radio sky, including the discovery of
Pulsars. Despite these early successes, parabolic dishes largely replaced
aperture arrays through the 1960's, and right up until the end of the 20th
century. Technological advances in areas such as signal processing, digital
electronics, low-power/high performance super-computing and large capacity data
storage systems have recently led to a substantial revival in the use of
aperture arrays - especially at frequencies below 300 MHz. Composed of simple
antennas with commercially available low-noise room-temperature amplifiers,
aperture arrays with huge collecting areas can be synthesized at relatively low
cost. Multiple beams (or multiple fields-of-view) can be rapidly formed and
electronically steered across the sky. As astronomers begin to grapple with
these new possibilities, the next goal is to see these systems move to higher
GHz frequencies. Aperture Arrays operating at frequencies of up to 1.7 GHz are
expected to form a substantial part of the Square Kilometre Array (SKA).
