  This is a review on cosmological perturbation theory. After an introduction,
it presents the problem of gauge transformation. Gauge invariant variables are
introduced and the Einstein and conservation equations are written in terms of
these variables. Some examples, especially perfect fluids and scalar fields are
presented in detail. The generation of perturbations during inflation is
studied. Lightlike geodesics and their relevance for CMB anisotropies are
briefly discussed. Perturbation theory in braneworlds is also introduced.
