  It is necessary to make assumptions in order to derive models to be used for
cosmological predictions and comparison with observational data. In particular,
in standard cosmology the spatial curvature is assumed to be constant and zero
(or at least very small). But there is, as yet, no fully independent constraint
with an appropriate accuracy that gaurentees a value for the magnitude of the
effective normalized spatial curvature $\Omega_{k}$ of less than approximately
$0.01$. Moreover, a small non-zero measurement of $\Omega_{k}$ at such a level
perhaps indicates that the assumptions in the standard model are not satisfied.
It has also been increasingly emphasised that spatial curvature is, in general,
evolving in relativistic cosmological models. We review the current situation,
and conclude that the possibility of such a non-zero value of $\Omega_k$ should
be taken seriously.
