  Modifications to gravity can provide attractive alternatives to the dark
components of the standard model of cosmology. These modifications to general
relativity (GR) must be hidden at small scales where theory is well tested, and
so one naturally looks to the large scales in order to detect any deviations
from GR. One particularly promising avenue in testing gravity at cosmological
scales is within the anisotropy of galaxy clustering in redshift space. This
thesis presents a framework for consistently constructing large scale structure
observables in redshift space for gravitational theories that include an
additional scalar degree of freedom, specifically, the Horndeski class of
theories with a generalized potential term. The relevance of such a framework
in the context of next generation spectroscopic surveys is then investigated
using N-body simulations. The thesis concludes with ongoing and recently
completed extensions to this framework, including interacting dark energy
models and the effective field theory of large scale structure.
