This paper describes an ongoing experiment at the Swiss Federal Institute of Technology in Lausanne regarding the use of World Wide Web-based document annotations for educational purposes. An annotation tool called Medium was built, merging database and Web technologies, to be used as a companion for classical university courses and collaborative learning experiments. Topics discussed include: (1) the concept of document annotations; (2) annotations versus news groups; (3) examples of other Web-based annotation tools; (4) project design constraints; (5) Medium's architecture; (6) the development process; (7) features and restrictions of Medium; (8) the user interface; (9) the evolution of Medium through three design phases; (10) extending Medium to add new functionalities; and (11) Medium as an education tool. Figures present the document refinement process, design phases, and cross-referencing documents. (Author/AEF)