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.�