The assumption of isostatic equilibrium is often used in geophysics; e.g., the depth of the lithosphere–asthenosphere boundary (LAB) is determined using data regarding the gravity field and topography, together with the assumption of isostasy. However, isostasy implies a hydrostatic state, which is contrary to the mantle convection hypothesis, which states that most of the mantle matter is in motion. We therefore discuss herein the question of when the assumption of isostasy can be used. It is suggested that isostasy may be used for parts of oceanic plates (except for subduction zones, hotspots, and oceanic ridges) and for many continental regions (except for postglacial regions and regions of intensive volcanic or tectonic activity). Moreover, using the results of a numerical model of convection and calculations of dynamic topography, it is shown that some generalization of isostasy is possible in the form of deep dynamic isostasy (DDI). It is also indicated that, for some regions without isostatic equilibrium (e.g., postglacial regions), it is possible to use the “isostatic” method with some corrections to the results.