Topic maps - the new ISO standard ISO-IEC 13250 - provide a bridge between the domains of knowledge representation and information management. Topics and topic associations build a structured semantic link network above information resources. Our research aims at visualizing this semantic layer efficiently, which is a critical issue as topic maps may contain millions of topics and associations. This paper is divided into 3 parts. First, we depict briefly basic topic maps concepts. Then, we review a few graph visualization techniques. Finally, we describe the visualization tool we developed at the Laboratoire d'Informatique de Paris 6 and study how this tool may be used - and enhanced - for topic maps visualization.