  The ``in-in'' formalism is reviewed and extended, and applied to the
calculation of higher-order Gaussian and non-Gaussian correlations in
cosmology. Previous calculations of these correlations amounted to the
evaluation of tree graphs in the in-in formalism; here we also consider loop
graphs. It turns out that for some though not all theories, the contributions
of loop graphs as well as tree graphs depend only on the behavior of the
inflaton potential near the time of horizon exit. A sample one-loop calculation
is presented.
