  We are at the dawn of a new golden age for radio astronomy, with a new
generation of facilities under construction and the global community focussed
on the Square Kilometre Array as its goal for the next decade. These new
facilities offer orders of magnitude improvements in survey speed compared to
existing radio telescopes and arrays. Furthermore, the study of transient and
variable radio sources, and what they can tell us about the extremes of
astrophysics as well as the state of the diffuse intervening media, have been
embraced as key science projects for these new facilities. In this paper we
review the studies of the populations of radio transients made to date, largely
based upon archival surveys. Many of these radio transients and variables have
been found in the image plane, and their astrophysical origin remains unclear.
We take this population and combine it with sensitivity estimates for the next
generation arrays to demonstrate that in the coming decade we may find
ourselves detecting 10^5 image plane radio transients per year, providing a
vast and rich field of research and an almost limitless set of targets for
multiwavelength follow up.

High Energy Astrophysical Phenomena