In reference to Bachelard’s theory, we use the adjective “phenomenotechnical” to reflect the a priori potentialities of a didactic theory, here the ATD, in revealing didactic phenomena. This study shows the type of phenomena that can be brought up to date through the construction of reference epistemological models (REM) and their confrontation with dominant epistemological models. The didactic problem of elementary differential calculus will be used as an illustration. After introducing the notion of phenomenotechnique and its relation to REMs, the chapter presents two REM proposals, one focused on elementary differential calculus and its connection to functional modelling, the second on the passage from differential calculus to analysis.