This chapter presents the theories to understand the properties of all nuclear ground states. These can be described either by discrete quantum numbers, e.g. number of neutrons and protons, angular momentum, parity and isospin, or by continuous variables like radius, binding energy and electric and magnetic moments. All these characteristics of the nucleus vary systematically through the periodic table. It describes a few of these quantities for stable nuclei and formulates the basic limits for stability to find the boundaries of the nuclear world. The spherically-symmetric single-particle potential leads to wave functions which can be chosen as eigenvectors for the z-component and square of the angular momentum operator. The saturation of the nuclear density, the large incompressibility and the roughly constant binding energy per unit volume are characteristics of a fluid. The semi-empirical liquid-drop-model mass formula is an extremely simple result of exploiting this analogy.