Primordial Black Holes (PBHs) are black holes formed in the early Universe, for instance from the collapse of large density perturbations generated by inflation.The discovery of gravitational waves from mergers of ~10 Solar mass black hole binaries has led to increased interest in PBHs as a dark matter candidate. I will review the formation of PBHs and the limits on their abundance, with particular emphasis on microlensing constraints in the Solar mass region. I will also emphasise key open question in the field (e.g. clustering, and methods for constraining asteroid mass PBHs).