ABSTRACT (Some key considerations, usually omitted, for a full understanding of the concept of Amount of substance) The understanding and correct use of the magnitude Amount of substance and its unit, the mole, presents serious difficulties, not only for students but even for teachers. Abundant research has shown the frequency of some erroneous conceptions shown by teachers and textbooks, which interpret amount of substance either as number of particles or, more frequently as mass¿ or even ignore the new magnitude, using the mole as 'atomic or molecular mass, given in grams'. Our aim here is to confront teachers' difficulties, discussing the ensemble of reasons that justify the necessity of introducing amount of substance. This complete analysis is something that has not been undertaken, to the best of our knowledge, in either textbooks or the abundant literature on this magnitude, thereby hindering the meaningful understanding of the concept.