An ultrasonic imaging technique has been developed and applied to examine smoldering combustion within a permeable medium. The technique provides information about local permeability variations within a smoldering sample, which can, in turn, be interpreted to visualize the propagation of the smolder reaction. The method utilizes the observation that transmission of an ultrasonic signal through a porous material increases with increasing permeability. Since a propagating smolder reaction leaves behind a char that is higher in permeability than the original (unburnt) material, ultrasonic transmission can be employed to monitor smolder progress. Additionally, the technique allows observation of the evolution of the char (i.e., material left by the smolder reaction), which: in certain circumstances, can continue to increase in permeability, due to secondary reactions (either oxidative or pyrolytic in nature). Experiments have been conducted, applying the technique to smoldering combustion in a two-dimensional geometry with line-of-sight imaging. For axisymmetric configurations. tomographic techniques have been implemented, providing three-dimensional mappings of the smolder front, as well as visualization of the smolder process itself. The results have furthered the understanding of two-dimensional smolder, and have been especially informative in identifying the controlling mechanisms leading to the transition from smoldering to flaming combustion.