After several decades of investigation, a surprisingly simple view of the lithospheric state of stress has emerged. Broad regions of the lithosphere (scales up to thousands of kilometers) are characterized by remarkably uniform stress fields, both in terms of orientations and relative magnitudes. There is a strong correlation between observed intraplate stress orientations and orientations predicted by models of the plate-driving forces acting on the plate geometry, suggesting that the same forces that drive the plates also stress their interiors. Lateral variations in density and thickness of the crust and lithosphere give rise to gravitational potential energy differences that locally induce forces comparable in magnitude to plate-driving forces. In addition, mantle density inhomogeneities may also influence lithospheric stresses through the linkage of mantle flow. Direct measurements of stress magnitudes at depth indicate that the brittle crust is in a state of ‘frictional faulting equilibrium’, that is, the stress differences are close to, and limited by, the stress levels required to induce slip on the most well-oriented pre-exising planes. This characteristic of ‘critically stressed crust’ stress applies equally well to active plate boundary regions as well as ‘stable’ intraplate regions, indicating that stress differences at depth are comparable in these very distinct tectonic provinces. The difference in deformation between the two provinces is simply the rate of deformation, primarily controlled by the integrated strength of the lithosphere. Warmer, weaker lithosphere (such as along plate boundaries) will deform about eight orders of magnitude more rapidly than old, cold lithosphere in the interior of plates.