The potential use of guided waves for defect characterization is studied. The influence of defect shape and size on transmitted and reflected fields is considered. Using the hybrid boundary element technique, the reflection and transmission coefficients for selected guided wave modes are numerically calculated and compared to experimental data. Selecting the aspect ratio as a shape parameter for various defects, the transmission and reflection coefficients are measured for certain guided wave modes input to the defect. The influence of defect size is then studied by monitoring the transmission and reflection coefficients for defects of various shapes and depths. The studies presented indicate that defect characterization is possible if a proper mode selection criteria can be established. The suitable features related to transmission and reflection coefficient data can also be used for algorithm development and implementation purposes of defect characterization.