The MATLAB Reservoir Simulation Toolbox is developed by SINTEF Applied Matemathics and is a result of our research on the development of new (multiscale) computational methodologies. The toolbox consists of a comprehensive set of routines and data structures for reading, representing, processing, and visualizing unstructured grids, with particular emphasis on the corner-point format used within the petroleum industry. The package is currently geared towards single- and two-phase flow and contains a set of mimetic and multiscale flow solvers and a few simple transport solvers capable of handling general unstructured grids. The toolbox is intended for rapid prototyping on complex (corner-point) grids and demonstration of the multiscale simulation concept. For more computationally challenging cases (e.g., models with more than 100 000 cells), we are developing a similar C++ toolbox, which is not yet publicly available.