The maintenance of large knowledge systems usually is a rather complex task. In this paper we will show that extensions or modifications of a knowledge base can be supported by appropriate visualizations techniques, e.g. by illustrating dependencies within the considered knowledge. In particular, we introduce a declarative approach for querying and visualizing rule–based knowledge represented as Xml documents; a knowledge engineer can extract and visually inspect parts of the knowledge base by ad–hoc declarations in a flexible manner.