  We consider the contribution of neutron stars and black holes to the
dynamical mass of galactic halos. In particular, we show that if these compact
objects were produced by an early generation of stars with initial metallicity
less than about $10^{-4} Z_\odot$, they can contribute at most 30--40% of the
Galactic halo mass without creating supersolar levels of enrichment. We show
that the case for halo neutron stars and black holes cannot be rejected on
metal overproduction arguments alone, due to the critical factor of the choice
of progenitor metallicity in determining the yields. We show that this scenario
satisfies observational constraints, similar to but no more severe than those
faced by halo white dwarfs. We also discuss the recent results on halo
microlensing, the presence of enriched hot gas in clusters and groups of
galaxies, and other observations. If there are halo neutron stars and black
holes, they will be detected by microlensing experiments in the future as
longer-timescale events.
