Knowledge engineering emerged as a very promising area to help improve software engineering practice. One of its possible applications would be to help in solving the numerous problems that affect the software maintenance activity. Maintainers of legacy systems developed years ago with obsolete techniques and tools, and not documented, need all kinds of knowledge (application domain, programming skills, software engineering techniques, etc.) It is generally assumed that formalizing all this knowledge and recording it would be a worthwhile effort. However, research is still in a early stage and numerous questions need to be answered: What knowledge should be targeted first? Where to find this knowledge? etc. To answer these questions, one needs a precise understanding of what knowledge is at stake here. We, therefore, propose an ontology of the knowledge needed to perform software maintenance. This ontology would be most useful as a framework for future research in knowledge engineering for software maintenance.