LIGO's first direct gravitational-wave detections have revived interest in an old dark-matter candidate, primordial black holes (PBHs). In this talk I will first discuss cosmic microwave background constraints to PBHs in the range of ~10 to a few hundred solar masses. I will then discuss PBH binary formation processes and the resulting merger rates. In particular, I will argue that LIGO may already set the most stringent limits on PBH abundance, provided PBH binaries formed in the early Universe are not strongly perturbed by tidal fields due to non-linear structures.