Research has shown that most of us who use traditional methods to teach physics in schools and universities are unable to change substantially our students' conceptual understanding. In an effort to address this problem, microcomputer-based laboratory (MBL) tools have been developed which interface to Apple II and Macintosh computers. Students use these tools to collect physical data which are graphed in real-time and then can be manipulated and analyzed. The MBL tools have made possible discovery-based laboratory curricula which embody results from educational research. In this paper, we describe a few of the tools (hardware and software)--the motion detector, the force probe, and the temperature probes--and introduce two curricula--kinematics and head and temperature. Students thrive in the MBL learning environment which is more like the working environment of practicing scientists than the traditional school environment. The curricula and tools allow students to take an active role in their learning and encourage them to construct knowledge from observation of the physical world. The ease of data collection and presentation encourage even badly prepared students to answer their own questions. The curricula take advantage of the fact that MBL tools present data in an immediately understandable graphical form to promote collaborative learning by encouraging peer discussions. We have used pre- and post-testing and observation to measure the conceptual understanding of secondary and university students in order to compare the MBL curricula to traditional methods. Students who use MBL materials show substantial and lasting understanding of basic physical concepts not learned by a majority of students in standard physical courses.