The propagation characteristics of Lamb waves in stiffened carbon fiber polymer matrix composite panels are investigated. In particular, the group velocities of S0 and A0 modes are calculated, the attenuation of Lamb waves induced by stiffeners and delaminations are evaluated, and the angular dependence of group velocity is subsequently examined. A two step damage detection and characterization approach, which uses both pulse-echo and pitch-catch active sensing schemes, is presented for the identification of delaminations in the stiffened composite panels. The matching pursuit decomposition algorithm is used for the representation of the sensor signal in the time-frequency domain. The delaminations are detected by identifying the converted Lamb wave modes in the time-frequency domain. The results confirm that the proposed approach is able to effectively identify the presence of seeded delaminations in complex composite structures.