A lecture given by Heung Nam Han, at the Adventures in the Physical Metallurgy of Steels (APMS) conference held in Cambridge University. About the nanoindentation of steel, and the pop-in effect, variant selection. Nano-indentation is an outstanding method to probe small-scale mechanical properties, which are relevant to a wide range of materials and applications. The response of a material to nano-indentation is usually presented in the form of a loadâ€"displacement curve. It is known that nano-indentation pop-in is a sudden displacement excursion on the load-displacement curve during load-controlled indentation. The sources of pop-in might be basically geometrical softening behaviors. In this study, several physical events which cause pop-ins during nano-indentation of steel alloys will be discussed. First, we consider the onset of plasticity resulting from dislocation nucleation or dislocation source activation in ferritic steel, which can produce the geometrical softening in the early stage of mechanical contact during nano-indentation. The effect of strain aging on the nano-indentation pop-in is observed and compared to the well-known macro-scale yield drop in tensile test. Second, both strain-induced alpha prime and epsilon martensitic transformations of metastable austenite are investigated by nano-indentation of individual austenite grains in multi-phase steels. The pop-ins are described as resulting from the geometrical softening due to the selection of favorable variants of alpha prime martensite and partial dislocation for epsilon martensite, respectively.