  As Cosmic Microwave Background (CMB) measurements are becoming more
ambitious, the issue of foreground contamination is becoming more pressing.
This is especially true at the level of sensitivity, angular resolution and for
the sky coverage of the planned space experiments MAP and PLANCK. We present in
this paper an indicator of the accuracy of the separation of the CMB
anisotropies from those induced by foregrounds. Of course, the outcome will
depend on the spectral and spatial characteristics of the sources of
anisotropies. We thus start by summarising present knowledge on the spectral
and spatial properties of Galactic foregrounds, point sources, and clusters of
galaxies. This information comes in support of a modelling of the microwave sky
including the relevant components. The accuracy indicator we introduce is based
on a generalisation of the Wiener filtering method to multi-frequency,
multi-resolution data. While the development and use of this indicator was
prompted by the preparation of the scientific case for the \plancks satellite,
it has broader application since it allows assessing the effective capabilities
of an instrumental set-up once foregrounds are fully accounted for, with a view
to enabling comparisons between different experimental arrangements. The real
sky might well be different from the one assumed here, and the analysis method
might not be in the end Wiener filtering, but this work still allow meaningful
{\em comparative} studies. As a matter of examples, we compare the CMB
reconstruction errors for the \maps and \plancks space missions, as well as the
robustness of the \plancks outcome to possible failures of specific spectral
channels or global variations of the detectors noise level across spectral
channels.
