Helicopter structures utilize materials with high specific strength and stiffness such as aluminum and composites. Adhesive bonding is a superior joining technique that saves weight, improves aerodynamics and reduces stress concentrations. However, adhesive joints are subject to invisible defects and need in-situ methods to monitor the structural integrity. Ultrasonic guided waves are famous for their large coverage area, capability of reaching inaccessible regions and have good sensitivity to many types of defects. Phased array transducers for structural health monitoring (SHM) are designed to make full use of the multimodal characteristic of guided waves. The unique damage indicators of mode amplitude ratio and wave package composition, are proposed after conducting theoretical and finite element analyses. Experiments are performed to verify the detection and characterization capability using proposed damage indicators. Good sensitivity and potential for defect sizing have been demonstrated for adhesive bonds for both aluminum and carbon fiber composite structures.