FEFLOW's theoretical basis is fully described in the comprehensive FEFLOW book.[1] It covers a wide range of physical and computational issues in the field of porous/fractured-media modeling. The book starts with a more general theory for all relevant flow and transport phenomena on the basis of the continuum mechanics, systematically develops the basic framework for important classes of problems (e.g., multiphase/multispecies non-isothermal flow and transport phenomena, variably saturated porous media, free-surface groundwater flow, aquifer-averaged equations, discrete feature elements), introduces finite element methods for solving the basic multidimensional balance equations, in detail discusses advanced numerical algorithms for the resulting nonlinear and linear problems, and completes with a number of benchmarks, applications and exercises to illustrate the different types of flow, mass and heat transport problems (e.g., subsurface flow and seepage problems, unsaturated-saturated flow, advective-diffusion transport, saltwater intrusion, geothermal and thermohaline flow).