  We analyze the oscillation signals generated by one extra sterile neutrino.
We fully take into account the effects of the established oscillations among
active neutrinos. We analyze the effects in solar, atmospheric, reactor and
beam experiments, cosmology and supernov\ae. We analyze the impact of the LSND
anomaly on cosmology showing, in the full 4 neutrino context, that it is not
compatible with standard cosmology. We identify the still allowed regions of
the parameter space, outlining which are the future experiments which can
improve the bounds.
