Ontology engineering is a relatively new and immature science. With new pos- sible applications and often non-normative standard proposals emerging rapidly it is hard to nd one universal formalization of ontology engineering rules. This causes many authors to fall into a trap of focusing on the application of an ontology and not giving enough attention to the ontology itself. Such approach results in ontologies that are hardly reusable, overcomplicated and di cult to understand which is the exact opposite of what a good ontology should be. While many problems are very speci c we attempt to bring the attention to a set of common ones with the intention of instructing how to x or avoid them. Here, we explain how we improved an existing ontology, Core Grid Ontology (CGO), that we use as an example. It is not our intention to focus on the quality of this ontology, which otherwise ts our needs well. Indeed, CGO was reused in the project Agents in Grid (AiG), where it was slightly modi ed then extended [1]. We build on that experience, starting with the reengineering of CGO that we will later extend with the improvements of the AiG ontology.