  The Cosmic Microwave Background fluctuations provide a powerful probe of the
dark ages of the universe through the imprint of the secondary anisotropies
associated with the reionisation of the universe and the growth of structure.
We review the relation between the secondary anisotropies and and the primary
anisotropies that are directly generated by quantum fluctuations in the very
early universe. The physics of secondary fluctuations is described, with
emphasis on the ionisation history and the evolution of structure. We discuss
the different signatures arising from the secondary effects in terms of their
induced temperature fluctuations, polarisation and statistics. The secondary
anisotropies are being actively pursued at present, and we review the future
and current observational status.
