  The future of cm and m-wave astronomy lies with the Square Kilometre Array
(SKA), a telescope under development by a consortium of 17 countries that will
be 50 times more sensitive than any existing radio facility. Most of the key
science for the SKA will be addressed through large-area imaging of the
Universe at frequencies from a few hundred MHz to a few GHz. The Australian SKA
Pathfinder (ASKAP) is a technology demonstrator aimed in the mid-frequency
range, and achieves instantaneous wide-area imaging through the development and
deployment of phased-array feed systems on parabolic reflectors. The large
field-of-view makes ASKAP an unprecedented synoptic telescope that will make
substantial advances in SKA key science. ASKAP will be located at the Murchison
Radio Observatory in inland Western Australia, one of the most radio-quiet
locations on the Earth and one of two sites selected by the international
community as a potential location for the SKA. In this paper, we outline an
ambitious science program for ASKAP, examining key science such as
understanding the evolution, formation and population of galaxies including our
own, understanding the magnetic Universe, revealing the transient radio sky and
searching for gravitational waves.
