  The effective evolution of an inhomogeneous universe model in Einstein's
theory of gravitation may be described in terms of spatially averaged scalar
variables. This evolution can be modeled by solutions of a set of Friedmann
equations for an effective scale factor, with matter and backreaction source
terms, where the latter can be represented by a minimally coupled scalar field
(`morphon field'). We review the basic steps of a description of backreaction
effects in relativistic cosmology that lead to refurnishing the standard
cosmological equations, but also lay down a number of unresolved issues in
connection with their interpretation within observational cosmology.
