TOUGH2 is a numerical simulator designed for simulating nonisothermal flows of multicomponent, multiphase fluids in porous and fractured media. It is primarily used in geothermal reservoir engineering, nuclear waste disposal, environmental assessment, and hydrology. The current version 2.0 includes enhanced fluid property modules, improved process modeling capabilities, and new user features such as enhanced linear equation solvers and graphics file output. The software is written in FORTRAN77 and can run on various platforms. This guide provides a comprehensive description of the code, including its methodology, architecture, array structure, input data preparation, and sample problems. TOUGH2 uses an integral finite difference method for discretizing space and time, and it offers flexible and transparent interfaces for different functional units. The guide also covers hardware requirements, installation instructions, and detailed explanations of input data formats and geometric data handling.TOUGH2 is a numerical simulator designed for simulating nonisothermal flows of multicomponent, multiphase fluids in porous and fractured media. It is primarily used in geothermal reservoir engineering, nuclear waste disposal, environmental assessment, and hydrology. The current version 2.0 includes enhanced fluid property modules, improved process modeling capabilities, and new user features such as enhanced linear equation solvers and graphics file output. The software is written in FORTRAN77 and can run on various platforms. This guide provides a comprehensive description of the code, including its methodology, architecture, array structure, input data preparation, and sample problems. TOUGH2 uses an integral finite difference method for discretizing space and time, and it offers flexible and transparent interfaces for different functional units. The guide also covers hardware requirements, installation instructions, and detailed explanations of input data formats and geometric data handling.