Advanced depth-sensitive hardness measurement equipment makes it possible to investigate the mechanical properties of solids under high local pressure at the nano-meter scale. During the depth-sensitive hardness measurement the diamond indenter pyramid is driven into the sample under continuous monitoring of load and penetration depth. The manufacture restrictions, which affect indenter tip rounding, (effective tip radius usually amounts about 100 nm) lead to the increase of the contact stress before the appearance of plasticity in partial dislocation-free surface of mono crystals. The discontinuity of the measured load-penetration depth curve is referred to as Pop-in-Effect. This effect is the result of the nucleation of the first dislocation loop and the subsequent material’s drastic response with plasticity to the applied stress. The Pop-in-effects were observed in metals (Al, Cu, Ni, T), ionic crystals (CaF2 ,BaF2) and semiconductors (CdTe, GaAs, GaP, InP, ZnSe). The mechanical stresses responsible for this process were estimated from the recorded experimental curve in the framework of elastic contact theory (Hertz, Sneddon). Within the isotropic approach the experimental results for loop nucleation measurements are in good agreement with the theory of dislocations. The observed stresses responsible for the generation of a stable dislocation loop yield up to G/9 (G is the shear modulus of the material). The corresponding dislocations were verified by means of microscopy imaging techniques (transmission electron microscopy (TEM), cathodoluminescence imaging (CL), and imaging of dislocation-etched surfaces).