[en] THETA is a computer simulation model for 3-dimensional coupled fluid, heat, and solute transport in porous media. The numerical solution used in the model is based on a finite difference approximation (FDM). For convective heat transport, an upwind scheme is used. THETA version 5.1 is a microcomputer version of the earlier main frame computer version of the model, developed also at the Helsinki University of Technology. Although some simplifications have been made, the essential features of the main frame version of THETA (e.g., fully homogeneous and anisotropic permeability) have been retained in the new version. THETA is intended for use in numerous applications of flow in porous media, such as aquifers (thermal energy storage, water supply, environmental issues) or fissured rock (nuclear waste disposal, gas storage) (15 refs., 13 figs.)