Indentation-induced structural phase transitions in single crystal Si(100) and amorphous silicon a-Si have been investigated for indentations made at room temperature and at 77 K. The experimental techniques employed were (1) Raman microscopy and (2) in situ electrical resistance measurement of the indentation region of the plastically deformed silicon. The Raman spectra from residual indentations revealed that although phase transitions did occur when indentations were made at room temperature, there were no phase transitions when indentations were made at 77 K. This difference in behaviour has been explained on the rise of temperature during the room temperature indentations, which may assist the phase transition process. The in situ electrical measurements have revealed that the deformed Si(100) yielded Ohmic behaviour, consistent with the view that during the indentation the cubic silicon transforms to the beta-Sn metallic phase (i.e. body-centre tetragonal).