We include gaseous continuum and line emission into our galev models for the spectral and photometric evolution of S imple S tellar P opulations ( SSP s ) for various metallicities in the range 0.02 \leq Z / Z _ { \odot } \leq 2.5 .
This allows to extend them to significantly younger ages than before .
They now cover the age range from 4 Myr all through 14 Gyr .
We point out the very important contributions of gaseous emission to broad band fluxes and their strong metallicity dependence during very early evolutionary stages of star clusters , galaxies or subgalactic fragments with vigorous ongoing star formation .
Emission-line contributions are commonly seen in these actively star-forming regions .
Models without gaseous emission can not explain their observed colors at all , or lead to wrong age estimates .
We use up-to-date Lyman continuum ( = Lyc ) emission rates and decided to use recent empirical determinations of emission line ratios relative to { H _ { \beta } } for subsolar metallicities .
We justify this approach for all situations where no or not enough spectral information is available to determine all the parameters required by photoionization models .
The effects of gaseous line and continuum emission on broad band fluxes are shown for different metallicities and as a function of age .
In addition to the many filter systems already included in our earlier models , we here also include the HST NICMOS and Advanced Camera for Surveys ( = ACS ) filter systems .