  This is a review of the theory of CMB anisotropies, an updated version of a
course given at the troisieme cycle de la Suisse Romande. An introduction to
gauge invariant cosmological perturbation theory is given and the theory CMB
anisotropies is develiped in this context. Simple analytical approximations for
the acoustic peak positions for adiabatic and isocurvature perturbations are
derived. Silk damping is discussed by an analytic approximation. A short
description of the present status of observations and parameter estimation
followed by a critical discussion terminate the review. The full system of
differential equations for CMB anisotropies and polarization needed in a
numerical treatment is also developed and given in an appendix.
