Despite the fact that the entire field of quantum mechanics was arguably started by Einstein's suggestion that light is quantized, it took several years before this idea was accepted, and, in fact, many more years before people could reliably produce single photons (and we are still working toward on-demand sources). I will discuss several approaches to single-photon sources, and several applications, including random number generation, quantum interrogation (the ability to "see" objects with no photons hitting them), and quantum computing. I will also describe proposals that may enable one to efficiently produce multiple photon states, and some of their applications as well.