Dynamic grain growth (DGG) has been observed in body-centered-cubic metals, including refractory metals. Dynamic grain growth occurs during concurrent plastic deformation at elevated temperatures, a feature that distinguishes it from static grain growth, which occurs during static annealing. Both normal and abnormal forms of dynamic grain growth occur. Dynamic abnormal grain growth (DAGG) was observed to create large grains in Mo and Ta, with some instances in Mo producing single crystals several centimeters long. Recent microstructural studies of BCC Fe and Mo materials suggest a mechanism by which dynamic grain growth may occur, a mechanism involving subgrains formed during deformation. Microstructural data and a theoretical basis for this possible mechanism will be presented and discussed.