The photon has become an important conceptual tool in current modern physics research related to quantum information and quantum computers. One might think that such a concept was settled through the theoretical and experimental achievements in quantum theory by the late 1920’s. However, the history of the concept of the photon is not so linear as it seems to be. In 1956 the experiment carried out by the British Robert Hanbury Brown (1916-2002) and Richard Quentin Twiss (1920-2005), for example, put the ‘traditional’ concept of the photon in question, from which the photon was represented as a billiard-ball picture. After the theoretical developments proposed by the American Nobel Prize Roy Glauber (1925- ), physicists started to discuss openly in the American Journal of Physics columns the meaning of the concept of the photon, trying to answer the questions – What is a photon? Is it a particle? Is it a wave? What is it? Another experimental result also contributed to the discussions on the photon concept in the 1980’s. Such an experiment was performed by the French physicist Alain Aspect (1947- ) and his research group who observed single-photons interferences, whose result was in agreement with the quantum description of the single-state photons. This article analyzes how physicists tried to answer those questions, considering their theoretical, experimental and philosophical arguments.