Non-destructive methods of determining the amount of decay in living trees are needed to quantify standing carbon reserves and parameterize cycling models.  Estimating decay volume in forests via external visual cues, (i.e. conks, broken branches, stem bulges, or stem discoloration) is often restricted to a certain species and sites.  Visual cues of rot in stems usually do not appear until the decay process is well underway. Ground-penetrating radar (GPR) has been identified as a non-destructive tool which may be useful for rapid detection of anomalies in both wood products and living trees.  We explore the range of detectable defects in several conifer (Pseudotsuga menziesii, Tsuga heterophyll, Thuja plicata, Tsuga canadensis, Pinus strobus) and hardwood species (Acer saccharum, Fagus grandifolia, Populus sp., Quercus rubra).  Prior work on conifers has indicated that hollow voids are detectable, however strong reflections from internal moisture gradients (wet sapwood:dry heartwood) can interfere with detection. Despite these problems, GPR can be applied to measure sapwood depth in healthy conifers. Preliminary findings with GPR in hardwood species indicate that a wider range of defects can be detected and accurately mapped than with conifers.