Beyond the power spectrum: primordial and secondary non-Gaussianity in
  the microwave background
  Cosmic microwave background observations are most commonly analyzed by
estimating the power spectrum. In the limit where the CMB statistics are
perfectly Gaussian, this extracts all the information, but the CMB also
contains detectable non-Gaussian contributions from secondary, and possibly
primordial, sources. We review possible sources of CMB non-Gaussianity and
describe statistical techniques which are optimized for measuring them,
complementing the power spectrum analysis. The machinery of $N$-point
correlation functions provides a unifying framework for optimal estimation of
primordial non-Gaussian signals or gravitational lensing. We review recent
results from applying these estimators to data from the WMAP satellite mission.
