  At present, there are several feasible observational probes of the first
stars in the universe. Here, we examine the constraints on early stellar
activity from the metallicity of the high-redshift Ly-$\alpha$ clouds, from the
effects of stellar ionizing photons on reionization and the cosmic microwave
background (CMB), and from the implications of gravitational microlensing
results for the presence of stellar remnants in galactic halos. We also discuss
whether the above signatures are consistent with each other, i.e., if they
reflect the same population of stars.
