  We review in a pedagogical way the present status of the impact of
non-Gaussianity (NG) on the Cosmic Microwave Background (CMB) anisotropies. We
first show how to set the initial conditions at second-order for the (gauge
invariant) CMB anisotropies when some primordial NG is present. However, there
are many sources of NG in CMB anisotropies, beyond the primordial one, which
can contaminate the primordial signal. We mainly focus on the NG generated from
the post-inflationary evolution of the CMB anisotropies at second-order in
perturbation theory at large and small angular scales, such as the ones
generated at the recombination epoch. We show how to derive the equations to
study the second-order CMB anisotropies and provide analytical computations to
evaluate their contamination to primordial NG (complemented with numerical
examples). We also offer a brief summary of other secondary effects. This
review requires basic knowledge of the theory of cosmological perturbations at
the linear level.
