  The quantity and quality of cosmic structure observations have greatly
accelerated in recent years. Further leaps forward will be facilitated by
imminent projects, which will enable us to map the evolution of dark and
baryonic matter density fluctuations over cosmic history. The way that these
fluctuations vary over space and time is sensitive to the nature of dark matter
and dark energy. Dark energy and gravity both affect how rapidly structure
grows; the greater the acceleration, the more suppressed the growth of
structure, while the greater the gravity, the more enhanced the growth. While
distance measurements also constrain dark energy, the comparison of growth and
distance data tests whether General Relativity describes the laws of physics
accurately on large scales. Modified gravity models are able to reproduce the
distance measurements but at the cost of altering the growth of structure
(these signatures are described in more detail in the accompanying paper on
Novel Probes of Gravity and Dark Energy). Upcoming surveys will exploit these
differences to determine whether the acceleration of the Universe is due to
dark energy or to modified gravity. To realize this potential, both wide field
imaging and spectroscopic redshift surveys play crucial roles. Projects
including DES, eBOSS, DESI, PFS, LSST, Euclid, and WFIRST are in line to map
more than a 1000 cubic-billion-light-year volume of the Universe. These will
map the cosmic structure growth rate to 1% in the redshift range 0<z<2, over
the last 3/4 of the age of the Universe.
