  The presence of cosmological fluctuations influences the background cosmology
in which the perturbations evolve. This back-reaction arises as a second order
effect in the cosmological perturbation expansion. The effect is cumulative in
the sense that all fluctuation modes contribute to the change in the background
geometry, and as a consequence the back-reaction effect can be large even if
the amplitude of the fluctuation spectrum is small. We review two approaches
used to quantify back-reaction. In the first approach, the effect of the
fluctuations on the background is expressed in terms of an effective
energy-momentum tensor. We show that in the context of an inflationary
background cosmology, the long wavelength contributions to the effective
energy-momentum tensor take the form of a negative cosmological constant, whose
absolute value increases as a function of time since the phase space of
infrared modes is increasing. This then leads to the speculation that
gravitational back-reaction may lead to a dynamical cancellation mechanism for
a bare cosmological constant, and yield a scaling fixed point in the asymptotic
future in which the remnant cosmological constant satisfies $\Omega_{\Lambda}
\sim 1$. We then discuss how infrared modes effect local observables (as
opposed to mathematical background quantities) and find that the leading
infrared back-reaction contributions cancel in single field inflationary
models. However, we expect non-trivial back-reaction of infrared modes in
models with more than one matter field.
