Expert systems are artificial intelligence applications that rely on a formalization of knowledge extracted from human experts. Expert systems had been extensively built and exploited in industries during the 1980s and early 1990s thanks to their performances which were comparable to that of human experts. Knowledge engineering is a discipline born for building knowledge bases suitable to expert systems. Although based on the Feigenbaum's principle “Knowledge is power”, knowledge engineering has been struggling to deliver due to lack of solid foundations. Ontology engineering, a discipline initiated by Nicola Guarino in the early 1990s, contributed to the provision of the needed foundations and to the innovation of knowledge engineering itself. This chapter gives a brief account of the influence of ontology engineering to knowledge engineering, in particular taking into account the Formal Ontology in Information Systems (FOIS) perspective as defined by Nicola in 1998. Some contributions made by his followers are also described.