Current mathematics instruction often fails to actively involve students. One way to address the problem is through the use of manipulatives, physical objects that help students visualize relationships and applications. We can now use computers to create virtual learning environments to address the same goals. This is a three-year NSF supported project to develop a library of uniquely interactive, web-based virtual manipulatives or concept tutorials, mostly in the form of Java applets, for mathematics instruction. The project is divided into these sections: Number & Operations; Algebra; Geometry; Measurement; Data Analysis & Probability. Within each, you will find a good selection of interactive applications for your K-12 students.