MidasPlus is an advanced molecular modeling system developed by the Computer Graphics Laboratory (CGL) at the University of California, San Francisco. The system is used daily in university-level research programs in order to display and manipulate macromolecules such as proteins and nucleic acids. Ancillary programs allow for such features as computation of molecular surfaces and electrostatic potentials and generation of publication quality space filling images with multiple light sources and shadows. MidasPlus is distributed as documented source code to serve as both a starting point and training tool for others interested in doing their own software development. To address the needs of MDI group's structure-based drug design program, MidasPlus has been developed with an emphasis on the interactive selection, manipulation and docking of drugs and receptors. Although quite powerful in this application, the system is also somewhat specialized in this re spect: it requires three dimensional atomic coordinate data for the structures being displayed and expects the primary structure to be based on linear chains of subunits such as amino acids or nucleic acids. Using MidasPlus for complex inorganic compounds or large polymers with many cross-links is discouraged. MidasPlus is now in use in nearly 400 other laboratories. Visitors from virtually every state and many foreign countries have published more than 1,200 scientific papers based on work done at the UCSF Computer Graphics Laboratory using MidasPlus and its predecessor MIDAS.