Octahedral void defects are widely recognized to be a cause of crystal-originated particles, flow-pattern defects, laser-scattering tomography defects, and oxide defects. Their structure has been characterized in detail and the dependence of their generation on Si growth conditions has been extensively examined. Void defects have been a major theme in the Si micro-device industry for several years and many unsolved problems still remain. This paper reviews this important field, and also presents some recent experimental results on void defects that we have obtained.