  The search for a quasi bound $\eta$ meson in atomic nuclei is reviewed. This
tentative state is studied theoretically as well as experimentally. The theory
starts from elastic $\eta$ nucleon scattering which is derived from production
data within some models. From this interaction the $\eta$ nucleus interaction
is derived. Model calculations predict binding energies and widths of the quasi
bound state. Another method is to derive the $\eta$ nucleus interaction from
excitation functions of $\eta$ production experiments. The $s$ wave interaction
is extracted from such data via final state interaction theorem. We give the
derivation of $s$ wave amplitudes in partial wave expansion and in helicity
amplitudes and their relation to observables. Different experiments extracting
the final state interaction are discussed as are production experiments. So far
only three experiments give evidence for the existence of the quasi bound
state: a pion double charge exchange experiment, an effective mass measurement,
and a transfer reaction at recoil free kinematics with observation of the decay
of the state.
