  The combination of mid-infrared data from the MSX satellite mission and
ground-based near-infrared photometry is used to characterise the properties of
the mid-infrared population of the Galactic plane. The colours of the youngest
sources still heavily embedded within their natal molecular clouds are in
general different from evolved stars shrouded within their own dust shells. Our
main motivation is to use MSX for an unbiased search for a large (~1000) sample
of massive young stellar objects (MYSOs). A simple analysis shows that the MSX
point source catalogue should contain most of the MYSOs within our Galaxy. We
develop colour selection criteria using combined near- and mid-infrared data
for MYSOs, which produces a list of 3071 objects, excluding the galactic centre
region. The programme of follow-up observations already underway to separate
the MYSOs from compact H II regions and other remaining objects is briefly
described. We also show that these data can be used, just as IRAS data has been
previously, to provide a separation between evolved stars with carbon rich and
oxygen rich dust. These data may also be used to search for evidence of dust
around normal main sequence stars, such as low mass pre-main sequence stars or
the Vega-excess class of objects where debris disks are presumed to remain from
the planet formation process. We discuss the accuracy and completeness of the
MSX point source catalogue, and show that the errors present tend to be of a
kind that is not significant for the main stellar populations we discuss in
this paper.
