Mechanistic modeling in animal science started in the 1980s with researchers from the USA, the UK, Australia, and New Zealand leading this. In France, Daniel Sauvant was a “driving force” in the development of a culture of modeling in research and education. For most animal scientists, modeling was perceived as somewhat mysterious, more related to complicated mathematics than to biology. An important contribution of Daniel has been the conceptualization of biological systems through an operating system (i.e., the flow of nutrients and metabolites through compartments) and a decisional system (the control of the flows). Furthermore, he integrated concepts proposed by Bauman and Currie of homeorhesis (a long-term trajectory) and homeostasis (short-term controls) in the models. The concept of a long-term trajectory was applied to protein and lipid deposition during the productive life of a growing animal, but also to the management of priorities during the entire life of an animal (e.g., to grow and mature, to reproduce and care for newborns, and eventually to die). In 1994, he stated that “to progress further in metabolic modelling, it will be necessary to have a better knowledge of the interaction of the metabolic effects of homeorhetic and homeostatic regulations”. He modeled this by comparing the actual situation of an animal with its position on the homeorhetic trajectory, and use the difference (e.g., due to nutrition or sanitary conditions) as a regulatory mechanism to return towards the trajectory. He applied the concepts an operating and a decisional system not only to animals, but also to livestock production systems in which homeorhesis can be perceived as a long-term goal of a farmer, and homeostasis as his management practices of the herd. Many of us who worked with Daniel may have been frustrated one day because our model did not predict what we expected, to which Daniel could reply “but in goes in the right direction”. There is probably not a better way to illustrate the limitations and potential of modeling in animal science. Daniel’s view to structure complicated and complex systems and his skills to makes this accessible have been a great source of inspiration for many of us.