Using advanced depth-sensitive hardness measurements, the homogenous nucleation of dislocations has been observed in dislocation-free single crystals. This process is related to a sudden displacement jump in the force-displacement curve. The mechanical stress for the set-in of this pop-in effect has been estimated with the Hertzian elastic contact theory. Experimental results of dislocation loop nucleation show good agreement with the continuum theory of dislocations. Electron microscopy provides a direct proof of dislocation nucleation during nanoindentation. PACS: 61.72.Lk, 81.40.Lm ?2003 The American Physical Society