  The era of the Square Kilometre Array is almost upon us, and pathfinder
telescopes are already in operation. This brief review summarizes our current
knowledge of extragalactic radio sources, accumulated through six decades of
continuum surveys at the low-frequency end of the electromagnetic spectrum and
the extensive complementary observations at other wavelengths necessary to gain
this understanding. The relationships between radio survey data and surveys at
other wavelengths are discussed. Some of the outstanding questions are
identified and prospects over the next few years are outlined.
