Advances in technology often rely on a world of photons as the basic units of light. Increasingly one reads of photons as essential to enterprises in Photonics and Quantum Technology, with career and investment opportunities. Notions of photons have evolved from the energy-packet crowds of Planck and Einstein, the later field modes of Dirac, the seeming conflict of wave and particle photons, to the ubiquitous laser photons of today. Readers who take interest in contemporary technology will benefit from learning what photons are now considered to be, and how our views of photons have changed -- in learning about the various operational definitions that have been used for photons and their association with a variety of quantum-state manipulations that include Quantum Information, astronomical sources and crowds of photons, the boxed fields of Cavity Quantum Electrodynamics and single photons on demand, the photons of Feynman and Glauber, and the photon constituents of the Standard Model
