“Is_A” links are the core component of all ontologies and are organized into “hierarchies of concepts”. In this paper we will first address the problem of an automatic help to build sound hierarchies. Dependencies called “existence constraints” are the foundation for the definition of a “normalized” hierarchy of concepts. In the first part of the paper algorithms are provided to obtain a normalized hierarchy starting either from concepts or from instances using Boolean functions. The second part of the paper is devoted to the hierarchy maintenance: automatically inserting, merging or removing pieces of knowledge. We also provide a way to give synthetic views of the hierarchy.