We present criteria for the analysis and elaboration of the content of instruction, at middle school level, in a specific domain: physics. This content, which we call the knowledge to be taught, has to respect several constraints in order to be teachable in a real school. Here we have chosen to analyse in depth those constraints which are related to the specific features of the physics. One of the constraints is a goal which, very often, justifies physics teaching: the operational aspect of physics knowledge in everyday life. The other constraint is related to a crucial aspect of an experimental science: the need for coherence between the formal aspects and their field of applicability. Our analysis is based on an epistemological approach to physics knowledge and on assumptions about learning. This analysis is related to the case of an innovative programme of teaching on electricity. Physics instruction from epistemological and didactical bases. Available from: https://www.researchgate.net/publication/226632253_Physics_instruction_from_epistemological_and_didactical_bases [accessed Apr 25, 2015].