  A variety of indices have been proposed in order to represent the many
different observables modulated by the solar cycle. Most of these indices are
highly correlated with each other owing to their intrinsic link with the solar
magnetism and the dominant eleven year cycle, but their variations may differ
in fine details, as well as on short- and long-term trends. In this paper we
present an overview of the indices that are often employed to describe the many
features of the solar cycle, moving from the ones referring to direct
observations of the inner solar atmosphere, the photosphere and chromosphere,
to those deriving from measurements of the transition region and solar corona.
For each index, we summarize existing measurements {\bf and typical use}, and
for those that quantify physical observables, we describe the underlying
physics.
