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The Medium Deep Survey is a HST Key Project which relies exclusively on parallel observations taken with the WFPC2 in $V$ and $I$. The parallel fields typically contain several hundred galaxies per field and the MDS is a statistical survey of these objects. Exposures range from one orbit to about ten or twenty orbits in the most favorable cases. The most interesting initial result is the relatively high proportion ($\sim$40\%) of objects which are in some way anomalous, and which probably explain the origin of the familiar excess population of faint (blue) galaxies found by others. The MDS has established the diversity of these faint objects, which are dominated by late-types and irregulars. But they also include apparently interacting pairs, compact galaxies, galaxies with superluminous starforming regions, diffuse low surface brightness galaxies of various forms, and galaxies with unresolved nuclei. Ground-based follow-up spectroscopy is being used to establish the space density and luminosities of these different classes. Cycle 4 results thus show that galaxy classifications can be reliably performed to 22 mag in the I band, where published spectroscopic surveys indicate a mean redshift of about 0.5. To this limit, the rest (about 60 \%) of the galaxies appear to be similar to regular Hubble-sequence examples observed at low redshift, with the relative proportion of spheroidal and disk systems as expected from nearby samples, indicating that the bulk of the local (giant) galaxy population was in place at half the Hubble time. Basic statistical data on galaxies can be extracted to fainter magnitudes of I = 24 - 25 or V = 25 - 26.
The Medium Deep Survey is supported by NASA/HST grants GO-2684 et seq.
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