  Emission lines in ionized nebulae can provide strong and useful constraints
on the properties of both ionizing and non-ionizing stellar populations in
regions with star formation, provided that stellar evolution and stellar
atmosphere models can be used in a selfconsistent way. Recently, the
application of these techniques has shown important discrepancies between
predicted and observed nebular spectra that point to stellar atmosphere models
of WR stars which are too energetic and/or to a significant leakageof high
energy photons. In this contribution, these two alternatives are analyzed in
detail.
