We present observations of optical second harmonic generation (SHG) from silicon nanocrystals embedded in SiO2. SHG sensitivity to Si/SiO2 interface states, charge on the nanocrystals, and particle density gradients is demonstrated. (C) 2000 Optical Society of America. OCIS codes: (190.3970) Microparticle nonlinear optics; (320.7110) Ultrafast nonlinear optics.