Of all the processes that might produce correlations among galaxies in our Universe, we know only one that definitely exists. It is gravity. Inevitably and inexorably, as Newton told Bentley, gravitational instability causes the galaxies to cluster.

Of all the descriptions of galaxy clustering, the correlation functions are connected most closely to the underlying gravitational dynamics. The next several chapters develop this connection. It resembles a great fugue, starting with simple themes and variations, then combining, developing, and recombining them to obtain a grand theoretical structure whose main insights have formed over the last three decades and which still continues to expand.