Why do we believe that the electromagnetic (em) field is quantized, and photons exist?   This talk will focus on two different ways that the quantization of the em field manifests itself in interference experiments. 
 
   Bohr, who initially doubted the reality of photons -- after Einstein's explanation of the photoeffect -- eventually proposed a thought experiment showing that the consistency of elementary quantum mechanics at the level of two slit diffraction requires quantizing the em field.  In modern terms, his argument implies that the em field must be able to store quantum information. 
 
   Yet another path to photons is through Hanbury Brown-Twiss interferometry and the closely related Hong-Ou-Mandel (HOM) effect.  Although both phenomena show interference at the classical level, anti-bunching in these effects when done with very weak fields implies that the em field cannot be classical.