Book excerpt: The digitalization of the energy sector is of paramount importance and, at the same time, an ever-increasing phenomenon. For example, it is worth noting how IoT technologies and the widespread utilization of digital twins are changing the way in which energy systems and fluid machinery are conceived and designed. However, this can only be achieved if these new ICT technologies are posed on solid bases for the representation of energy systems and fluid machinery. Therefore, mathematical modelling is still relevant, and its importance cannot be underestimated. This Special Issue, which follows a previous one on the same topic, is intended for a collection of contributions about the mathematical modelling of energy systems and fluid machinery in order to build and consolidate this base of knowledge.