  Significant new opportunities for astrophysics and cosmology have been
identified at low radio frequencies. The Murchison Widefield Array is the first
telescope in the Southern Hemisphere designed specifically to explore the
low-frequency astronomical sky between 80 and 300 MHz with arcminute angular
resolution and high survey efficiency. The telescope will enable new advances
along four key science themes, including searching for redshifted 21 cm
emission from the epoch of reionisation in the early Universe; Galactic and
extragalactic all-sky southern hemisphere surveys; time-domain astrophysics;
and solar, heliospheric, and ionospheric science and space weather. The
Murchison Widefield Array is located in Western Australia at the site of the
planned Square Kilometre Array (SKA) low-band telescope and is the only
low-frequency SKA precursor facility. In this paper, we review the performance
properties of the Murchison Widefield Array and describe its primary scientific
objectives.
