In the context of exploring educational hypermedia on the World Wide Web, this paper presents a history visualization that helps learners reorganize explored contents to find what Web sites they should visit next. Learning-by-exploration is discussed as it relates to educational hypermedia, including the "Get Lost in Hyperspace" problem, learning support approaches, and current work on hypermedia exploration aids. A history visualization tool (HiVis) is presented. The HiVis user interface and three types of visualization that are not currently implemented in standard Web browsers are described: (1) time sequential representation with contextual information; (2) structural representation; and (3) superposition on an overview diagram. A preliminary evaluation of HiVis is summarized; results indicate that the history visualization is informative for learners. Figures present the HiVis interface, a time sequential representation, structural representation, and superposition representation. Contains 11 references. (Author/AEF)