  The analysis of anisotropies in the cosmic microwave background (CMB) has
become an extremely valuable tool for cosmology. We even have hopes that
planned CMB anisotropy experiments may revolutionize cosmology. Together with
determinations of the CMB spectrum, they represent the first cosmological
precision measurements. This is illustrated in the talk by Anthony Lasenby. The
value of CMB anisotropies lies to a big part in the simplicity of the
theoretical analysis. Fluctuations in the CMB can be determined almost fully
within linear cosmological perturbations theory and are not severely influenced
by complicated nonlinear physics. In this contribution the different physical
processes causing or influencing anisotropies in the CMB are discussed. The
geometry perturbations at and after last scattering, the acoustic oscillations
in the baryon-photon-plasma prior to recombination, and the diffusion damping
during the process of recombination. The perturbations due to the fluctuating
gravitational field, the so called Sachs-Wolfe contribution, is described in a
very general form using the Weyl tensor of the perturbed geometry.
