Knowledge of the Earth’s gravity field is an essential component for understanding the physical system of the Earth. Inside the masses, the field interacts with many other fields, according to complicated processes of physical and chemical nature; the study of these phenomena is the object of geophysics. Outside the masses, the gravity field smoothes out in agreement with the “harmonic” character of gravitation, while preserving, particularly close to the Earth’s surface, the signature of the internal processes; the study of the gravity field on the boundary and in the external space is the object of physical geodesy. It is necessary to define a separation surface between the masses and the “free” space. This surface is the geoid, an equipotential surface of the gravity field in a stack of such surfaces, close to the surface of the sea. Determining the geoid, or some other surface closer to the Earth's surface, has become synonymous to modelling the gravity field in physical geodesy; this is the subject of this book. Nowadays, this knowledge has become a practical issue also for engineering and other applications, because the geoid is used as a reference surface (datum) of physical heights that is very important in order to relate such heights to purely geometric ones obtained, for example, from GNSS. The methods currently used to produce the geoid at the centimetre level require significant mathematical, stochastic and numerical analysis. The book is structured in such a way as to provide self consistently all the necessary theoretical concepts, from the most elementary ones, such as Newton’s gravitation law, to the most complicated ones dealing with the stability of solutions of boundary value problems. It also provides a full description of the available numerical techniques for precise geoid and quasi-geoid determination. In this way, the book can be used by both students at the undergraduate and graduate level, as well as by researchers engaged in studies in physical geodesy and in geophysics. The text is accompanied by a number of examples, from most elementary to more advanced, as well as by exercises that illustrate the main concepts and computational methods.