The purpose of this paper is to discuss how general problem-based learning (PBL) models and social-constructivist perspectives are applied to the design and development of a Web-based science program, which emphasizes inquiry-based learning for fifth grade students. The paper also deals with the general features and learning process of a Web-based science program, including the teacher's action, the student's learning activity, and cognitive tools to scaffold the student's inquiry. A major goal of PBL in the Web-based science program is to help students develop scientific thinking and problem solving skills through a set of interaction tools (teacher-to-student, student-to-student, group-to-group, student-scientist, and student-cognitive). The main stages of PBL in the Web-based science program are: (1) "Your Challenge," in which students engage in an authentic problem situation; (2) "Plan Inquiry," in which students identify what they know, what they need to know to solve a problem, and how they go about finding out; (3) "Explore Resources," in which students gather information, learning primary concepts and principles necessary to solve the problem, and contact a scientist to acquire a scientific thinking process for solving problems and their perspective about the problem; (4) "Generate Alternative Solutions," in which students come up with alternative solutions to the problem after analyzing information and data; and (5) "Reflection and Presentation," which focuses on feedback from the teacher, students, and scientist about alternative solutions. After revising their solutions, students present their best solutions. In this learning cycle, scaffolding or cognitive tools support students' activities. (Author/AEF)