In order to understand fluid flows in underground porous formations, engineers need to produce models, in the form of grid systems populated with physical properties such as permeability and porosity. This procedure is of crucial importance but it is also problematic. It is crucially important in determining where and how fluids flow; reservoir or aquifer modeling is used to plan field development, optimize oil production with the judicious selection of well locations, assess contaminant migration, design capture zones, and so on. It is problematic, because there is never enough data available to describe with certainty the spatial distribution of permeability and porosity on a given scale. Given the complex heterogeneity of natural porous media, the fundamental question is: how can this reality be incorporated in models? This textbook refers to geostatistics and optimization to review the whole workflow for modern reservoir characterization and to provide an original solution. A CD-ROM with a software called GO is supplied with this book. It provides tools to answer the illustrative exercises proposed and to help the reader to develop intuitive understanding. This book is written at a comprehensible level for students who have had calculus, linear algebra and some exposure to differential equations. It should also serve already-practicing engineers in oil reservoirs, environment and hydrology.