  (first paragraph) We have obtained Halpha and Palpha emission line images
covering the central 3 - 4 arcmin of M51 using the WFPC2 and NICMOS cameras on
HST to study the high-mass stellar population. The 0.1 - 0.2 arcsec pixels
provide 4.6 - 9 pc resolution in M51 and the Halpha/Palpha line ratios are used
to obtain extinction estimates. A sample of 1373 Halpha emission regions is
catalogued using an automated and uniform measurement algorithm. Their sizes
are typically 10 - 100 pc. The luminosity function for the Halpha emission
regions is obtained over the range L_{Halpha} = 10^{36} to 2 times 10^{39} erg
s{-1}. The luminosity function is fit well by a power law with dN/dlnL
proportional to L^{-1.01}). The power law is significantly truncated and no
regions were found with observed L_{Halpha} above 2 times 10^{39} erg s^{-1}
(uncorrected for extinction; the maximum seen in ground-based studies is
approximately a factor of 5 higher, very likely due to blending of multiple
regions). The extinctions derived here increase the maximum intrinsic
luminosity to above 10^{40} erg s^{-1}). The logarithmically binned luminosity
function is also somewhat steeper (alpha = -1.01) than that found ground-based
imaging (alpha = -0.5 to -0.8)
  - probably also a result of our resolving regions which were blended in the
ground-based images. The 2-point correlation function for the HII regions
exhibits strong clustering on scales <= 2 arcsec or 96 pc.
