Book excerpt: Geoscientists and Engineers taking an interest in Petrophysics, are struck by the contrasting treatment of the Physics Aspects and the Geology Aspects. If we are to scale up isolated petrophysical observations to an entire oil reservoir or an aquifer, it is essential to implement the powerful extrapolation tool of geological interpretation. This is clearly based on a good understanding of the relations between the petrophysical parameters studied and the petrological characteristics of the rock considered. The book is divided into two sections of different size. The first section (by far the largest) describes the various petrophysical properties of rocks. Each property is defined, limiting the mathematical formulation to the strict minimum but emphasising the geometrical and therefore petrological parameters governing this property. The second section concentrates on methodological problems and concerns, above all, the representativeness of the measurements and the size effects. The notions of Representative Elementary Volume, Homogeneity, Anisotropy, RockType, etc. provide a better understanding of the problems of up-scaling (Plug, Core, Log Analysis, Well Test). Lastly, we provide a description of several Porous Network investigation methods:Thin section, Pore Cast, Visualization of capillary properties, X-ray tomography.