Many studies on misconceptions in physics and in physics teaching have shown that both students and teachers have misconceptions about such important concepts as mass, energy, heat, etc. The author of this communication has made an exhaustive research into conceptions of mass both in the history of Physics and in Physics teaching. He found some historical and epistemological reasons for the multiple answers to questions like these: • What is the scientific meaning of mass? • Does the mass of a particle depends on its velocity? • Is the mass of a particle equivalent to its total energy? • In an isolated system, could there be conversion of mass into energy or energy into mass? • Is the total mass of a system the sum of their particles' mass? Etc. The objective of this communication is to give the results of the research developed on the teachers' ideas about mass, and to discuss the historical and epistemological reasons why some of them are misconceptions, using a methodology based on heuristic tools.