  Inflationary predictions for the power spectrum of the curvature perturbation
have been verified to an excellent degree, leaving many models compatible with
observations. In this thesis we studied third-order correlations, that might
allow one to further distinguish between inflationary models. From all the
possible extensions of the standard inflationary model, we chose to study
two-field models with canonical kinetic terms and flat field space. The new
feature is the presence of the so-called isocurvature perturbation. Its
interplay with the adiabatic perturbation outside the horizon gives birth to
non-linearities characteristic of multiple-field models. In this context, we
established the second-order gauge-invariant form of the adiabatic and
isocurvature perturbation and found the third-order action that describes their
interactions. Furthermore, we built on and elaborated the long-wavelength
formalism in order to acquire an expression for the parameter of
non-Gaussianity fNL as a function of the potential of the fields. We next used
this formula to study analytically, within the slow-roll hypothesis, general
classes of potentials and verified our results numerically for the exact
theory. From this study, we deduced general conclusions about the properties of
fNL, its magnitude depending on the characteristics of the field trajectory and
the isocurvature component, as well as its dependence on the magnitude and
relative size of the three momenta of which the three-point correlator is a
function.
