  This review deals with properties of very young (< 10-20 Myr) stellar
clusters which are in the nebular phase and are embedded in photoionized
regions (classical extragalactic HII regions (RHIIs) or starburst galaxies).
Based on the analysis of the integrated light of these clusters at the UV and
optical wavelengths, different techniques are discussed that allow to estimate
in consistent the stellar content and the evolutionary state of the ionizing
clusters.
