Primordial Black Holes

In this chapter we first describe the early history of primordial black hole (PBH) research. We then discuss their possible formation mechanisms, including critical collapse from inflationary fluctuations and various types of phase transition. We next describe the numerous constraints on the number density of PBHs from various quantum and astrophysical processes. This was the main focus of research until recently but there is currently a shift of emphasis to the search for evidence for PBHs. We end by discussing this evidence, with particular emphasis on their role as dark matter candidates, sources of gravitational waves and seeds for supermassive black holes and early cosmic structures.