The Southern Stellar Stream Spectroscopic Survey (${S}^5$): Overview,
  Target Selection, Data Reduction, Validation, and Early Science
  We introduce the Southern Stellar Stream Spectroscopy Survey (${S}^5$), an
on-going program to map the kinematics and chemistry of stellar streams in the
Southern Hemisphere. The initial focus of ${S}^5$ has been spectroscopic
observations of recently identified streams within the footprint of the Dark
Energy Survey (DES), with the eventual goal of surveying streams across the
entire southern sky. Stellar streams are composed of material that has been
tidally striped from dwarf galaxies and globular clusters and hence are
excellent dynamical probes of the gravitational potential of the Milky Way, as
well as providing a detailed snapshot of its accretion history. Observing with
the 3.9-m Anglo-Australian Telescope's 2-degree-Field fibre positioner and
AAOmega spectrograph, and combining the precise photometry of DES DR1 with the
superb proper motions from $Gaia$ DR2, allows us to conduct an efficient
spectroscopic survey to map these stellar streams. So far ${S}^5$ has mapped 9
DES streams and 3 streams outside of DES; the former are the first
spectroscopic observations of these recently discovered streams. In addition to
the stream survey, we use spare fibres to undertake a Milky Way halo survey and
a low-redshift galaxy survey. This paper presents an overview of the ${S}^5$
program, describing the scientific motivation for the survey, target selection,
observation strategy, data reduction and survey validation. Finally, we
describe early science results on stellar streams and Milky Way halo stars
drawn from the survey. Updates on ${S}^5$, including future public data
release, can be found at \url{http://s5collab.github.io}.
