  The CMB bispectrum is a potential window on exciting new physics, as it is
sensitive to the non-Gaussian features in the primordial fluctuations, the same
fluctuations that evolved into today's planets, stars and galaxies. However,
this invaluable information is potentially screened, as not all of the observed
non-Gaussianity is of primordial origin. Indeed, a bispectrum arises even for
perfectly Gaussian initial conditions due to non-linear dynamics, such as CMB
photons scattering off free electrons and propagating in an inhomogeneous
Universe. In this thesis, I introduce the reader to this intrinsic bispectrum
in a pedagogic way, building up from the standard model of cosmology and from
cosmological perturbation theory, the tool cosmologists use to unravel the
history of the cosmos. In doing so, I introduce $\text{SONG}$, a new and
efficient code for solving the second-order Einstein and Boltzmann equations.
Next, I move on to answer the crucial question: is the intrinsic bispectrum
going to screen the primordial signal in the CMB? Using $\text{SONG}$ I compute
the intrinsic bispectrum and shows how its contamination leads to a small bias
in the estimates of primordial non-Gaussianity, a great news for the prospect
of using CMB data to probe primordial non-Gaussianity.
