TOUGH2 is a numerical simulator for nonisothermal flows of multicomponent, multiphase fluids in porous and fractured media. It is used in geothermal reservoir engineering, nuclear waste disposal, environmental assessment, and hydrology. The first version was released in 1991, with updates in 1994. Version 2.0 includes new fluid property modules, enhanced process modeling, and improved user features. TOUGH2 is written in standard FORTRAN77 and can run on various platforms with appropriate compilers. It provides a technical description of the code, including physical processes, mathematical methods, and sample problems. The report includes a table of contents, list of figures, and tables, along with an abstract, introduction, hardware requirements, methodology, array structure, input data preparation, equation-of-state modules, user features, output, sample problems, and references. The document includes detailed information on the code's structure, input data formats, and various modules for different fluid and heat flow scenarios. It also discusses the program's architecture, numerical methods, and input data preparation. The report provides a comprehensive guide to using TOUGH2 for subsurface flow problems.TOUGH2 is a numerical simulator for nonisothermal flows of multicomponent, multiphase fluids in porous and fractured media. It is used in geothermal reservoir engineering, nuclear waste disposal, environmental assessment, and hydrology. The first version was released in 1991, with updates in 1994. Version 2.0 includes new fluid property modules, enhanced process modeling, and improved user features. TOUGH2 is written in standard FORTRAN77 and can run on various platforms with appropriate compilers. It provides a technical description of the code, including physical processes, mathematical methods, and sample problems. The report includes a table of contents, list of figures, and tables, along with an abstract, introduction, hardware requirements, methodology, array structure, input data preparation, equation-of-state modules, user features, output, sample problems, and references. The document includes detailed information on the code's structure, input data formats, and various modules for different fluid and heat flow scenarios. It also discusses the program's architecture, numerical methods, and input data preparation. The report provides a comprehensive guide to using TOUGH2 for subsurface flow problems.