  We review the observations of nebular HeII 4686 emission in Local Group
objects and extra-galactic HII regions. Different ionization mechanisms
proposed to explain the unusually high excitation are summarized. The models of
Schaerer (1996) and Schaerer & Vacca (1998) predict nebular HeII emission due
to hot WN and WC/WO stars, in good agreement with observations of Wolf-Rayet
(WR) galaxies showing WR signatures and/or nebular HeII. Comparisons of
synthesis models with observations suggest that nebular HeII emission may be
related to WR stars in all cases. The case of I Zw 18 is discussed. It is shown
that the observed WR lines and nebular HeII can be reproduced with an
instantaneous burst model and a Salpeter IMF. This supports the suggestion that
WR stars are responsible for nebular HeII emission.
