The understandings we associate with quantum theory began a little over a hundred years ago with the discovery that light is divided into quanta that behave in some contexts like particles. When elementary particles were presently discovered to share the wave-like properties of light, the stage was set for the development of quantum mechanics. It was not until 1927 however, when the mathematics developed to describe the new theory was applied to the electromagnetic field, that we finally had a self-consistent theory of light quanta. The many detailed verifications of the theory that followed, were mainly confined to radiation processes involving only one or two light quanta. The theory of multi-quantum phenomena began to expand rapidly in the 1950s, and has been greatly accelerated by the development of the laser. We shall review the quantum theoretical meaning of optical coherence and discuss its relation to the statistical distributions of light quanta and to recent observations of the behavior of individual quanta.