Black hole mergers along the cosmic history

Galaxy mergers are an integral part of the structure evolution scenario. Massive black holes have inhabited the centers of galaxies since early cosmic times, when we detect them as the engines of luminous quasars. Mergers are therefore expected from the evolving population of massive black holes along the hierarchy of galaxy formation. I will discuss the expected properties of massive black hole mergers from early times to the present, and describe observational diagnostics that can shed light on the history of massive black holes.