An imaging algorithm based on time reversal was developed to detect debonding in sandwich composite structures using guided ultrasonic wave signals from a surface-attached active sensor network. Some fundamentals associated with the time reversal of guided ultrasonic waves in sandwich carbon fiber/epoxy composite panels are studied. The reversibility of fundamental symmetric and antisymmetric modes were investigated when they excited at two different central frequencies, and their sensitivity to anomalies (such as debonding) in composite sandwich structures was investigated. The correlation coefficients between the original and reconstructed time-reversed signals in the time or frequency domains were calculated to define the damage index for individual sensing paths, which were used to develop an imaging algorithm to identify the presence of damage in the monitoring area enclosed by the active sensor network. The results confirm that incident wave signals and their reconstructed time-reversed counterparts can be used to accurately detect damage in sandwich composite structures.