  We give a basic introduction to ghost-free nonlinear theories involving
massive spin-2 fields, focussing on bimetric theory. After motivating the
construction of such models from field theoretical considerations, we review
the linear theories for massive and massless spin-2 fluctuations propagating on
maximally symmetric backgrounds. The structure of general nonlinear spin-2
interactions is explained before we specialise to the ghost-free case. We
review the maximally symmetric solutions of bimetric theory, its mass spectrum
and the parameter limit which brings the theory close to general relativity.
Finally we discuss applications of bimetric theory to cosmology with particular
emphasis on the role of the general relativity limit.
