Amorphous solids increase their stress as a function of an applied strain until a mechanical yield point whereupon the stress cannot increase anymore, afterward exhibiting a steady state with a constant mean stress. In stress-controlled experiments, the system simply breaks when pushed beyond this mean stress. The ubiquity of this phenomenon over a huge variety of amorphous solids calls for a generic theory that is free of microscopic details. Here, we offer such a theory: The mechanical yield is a thermodynamic phase transition, where yield occurs as a spinodal phenomenon. At the spinodal point, there exists a divergent correlation length that is associated with the system-spanning instabilities (also known as shear bands), which are typical to the mechanical yield. The theory, the order parameter used, and the correlation functions that exhibit the divergent correlation length are universal in nature and can be applied to any amorphous solids that undergo mechanical yield.shear bands | yielding | glass | spinodal | criticality A solid, be it crystalline or amorphous, is operatively defined as any material capable to respond elastically to an externally applied shear deformation (1). However, any solid material, when subject to a large-enough shear strain, finally undergoes a mechanical yield. Here, we focus on the mechanical yield of amorphous materials, such as molecular and colloidal glasses, foams, and granular matter. The phenomenology exhibited by the yielding point within this vast class of materials, as reported in countless strain-controlled simulations (2-8) and experiments (9-11), shows a remarkable degree of universality, despite the highly varied nature of the model systems involved. Among these universal features is the presence, at the onset of flow at yielding, of system-spanning excitations referred to as shear bands (12, 13), wherein the shear strongly localizes, leaving the rest of the material unperturbed. This phenomenon is of capital importance for engineering applications, because it is responsible for the brittleness typical of glassy materials, in particular metallic glasses (14), whose potential for practical use is stymied by their tendency to shear band and fracture (13,15,16).In athermal amorphous solids, the phenomenon has universal features. For strains γ smaller than some critical value denoted as γY , the stress in the material grows on the average when the strain is increased. After yield, the stress cannot grow on the average, no matter how much the strain is increased. The universality of the basic phenomenology of yielding begs a picture of its characteristics in terms of a universal theory, in the sense that such a theory should rely on a statistical-mechanical framework and be independent of details, such as chemical composition and production process of the material. This need was addressed in a recent work (17), wherein building up from ideas first advanced in ref. 18, there emerged a picture of mechanical yielding as a first-order phenomenon [i.e., as a discontinuous ph...