Book excerpt: One important precondition for modeling multiphase flow and transport processes in the hydrosystem "subsurface" is the general formulation of a model. The objective of this book is to present a consistent, easily accessible formulation of the fundamental phenomena and concepts, to give a uniform description of mathematical and numerical modeling, and to show the latest developments in the field of simulation of multiphase processes, especially in porous and heterogeneous media. Some general aspects which affect the selection of the relevant processes and the corresponding parameters as well as the mathematical and numerical model concepts are discussed in detail.