  This PhD thesis presents a study of the effect and generation of
non-adiabatic perturbations in Cosmology. We study adiabatic (curvature) and
entropy (isocurvature) perturbations produced during a period of cosmological
inflation that is driven by multiple scalar fields with an arbitrary
interaction potential. A local rotation in field space is performed to separate
out the adiabatic and entropy modes. The resulting field equations show
explicitly how on large scales entropy perturbations can source adiabatic
perturbations if the background solution follows a curved trajectory in field
space, and how adiabatic perturbations cannot source entropy perturbations in
the long-wavelength limit. We show why one in general expects the adiabatic and
entropy perturbations to be correlated at the end of inflation. Then, we
consider two-field preheating after inflation, examining conditions under which
entropy perturbations can alter the large-scale curvature perturbation and
showing how our new formalism has advantages in numerical stability when the
background solution follows a non-trivial trajectory in field space. Then we
compare the latest cosmic microwave background data with theoretical
predictions including correlated adiabatic and CDM isocurvature perturbations.
We find that there is a degeneracy between the amplitude of correlated
isocurvature perturbations and the spectral tilt. We then study perturbations
in Randall-Sundrum-type brane-world cosmologies. The density perturbations
generate Weyl curvature in the bulk, which in turn backreacts on the brane via
stress-energy perturbations. Bulk effects produce a non-adiabatic mode, even
when the matter perturbations are adiabatic, and alter the background dynamics.
