This study investigated the use of an online authentic assessment tool to evaluate students' fluency with the World Wide Web (WWW). Participants included 120 middle and high school students with a strong technology background and a familiarity with navigating the World Wide Web. Using the online Web Expertise Assessment and accompanying WWW background questionnaire, students were able to demonstrate their WWW knowledge and expertise. Researchers then identified important individual measures and coded these reliably. Results from principal components factor analysis suggest four broad indicators of Web expertise: navigational strategies, prior Web knowledge, searching expertise, and finding ability. In general, these composite indicators make sense theoretically, and the results support the construct validity of each. The indicators appear to be reliable, with alphas ranging from 0.88 to 0.71. In addition, these measures match the theoretical conceptions grounded in the literature. The paper concludes with a discussion of implications for future research in assessing and supporting students' Web learning. (Contains 1 table, 4 figures, and 28 references.) (Author/SLD)