The number of researchers using computational chemistry tools is growing every year. There are multiple programs used for calculation of various molecular and electronic properties such as optimized geometry, energy, vibrational spectra, and so forth. Another set of programs is used for the visualization of these properties. However, such programs are either too complex for a beginner or too simple for an intermediate user for everyday use. Molecular Modeling and Simulation Kit (MaSK) is designed to fill this gap by presenting an easy-to-use intuitive interface to quantum chemical programs such as GAMESS and Gaussian with an array of advanced tools. The program can be used as a postprocessor to visualize calculated properties or as a preprocessor to prepare the input files for quantum chemical programs. In addition, some properties such as the surfaces of molecular orbitals, electron and spin densities, and molecular electrostatic potentials are actually calculated by MaSK. If MaSK is combined with PC GAMESS, the preparation of the input, running of the calculations, and the display of the results can all be done without leaving the program's interface. (C) 2008 Wiley Periodicals, Inc. Int J Quantum Chem 109: 8-16, 2009