Honeycomb sandwich structures (HSS), due to their high strength/stiffness to density ratio, are widely used in aerospace industry. Efficient and reliable techniques are required to detect damages in HSS. While conventional damage detection methods cannot meet the requirements, ultrasonic guided waves, due to their long propagating range and sensitivity to different types of defects, have the potential to significantly improve the state-of-the art in the NDE of HSS. However, the literature on ultrasound propagation in HSS is rather sparse, especially on the detailed characteristics of the waves and their interaction with realistic defects in HSS components. When the defect is smaller, excitation signal with higher frequency is required. However, there is no published research on this subject.The major objective of this research is to study high frequency wave propagation in HSS with or without the presence of defects. A numerical searcher is developed to solve the dispersion equations in HSS under very high frequency excitations. With this tool, solutions for high frequency Lamb waves in isotropic media are obtained and compared with numerical simulations. To obtain the solution in layered media, the global matrix method (GMM) is used and solutions for force responses are obtained by the residue theorem. However, the conventional method has singularity issues causing numerical instability in certain frequencies. A new formulation of the method is developed to solve the problem. The HSS is later homogenized to a three-layered medium and the dispersion curves for this model is studied with changes in the core material properties. At the end, the conventional damping model for Lamb waves is validated through the method and the corresponding damping coefficients are obtained experimentally. An experiment is performed to obtain the signal features when disbond damages are introduced in the HSS. The characteristics of the waves incident from different angles are studied to identify the most favorable incident angle for defects detection Based on this study, a new damage index is extracted from the signals and is applied to improve the quality of the damage detection images. Finally, an experiment is performed about the feasibility of using non-contact transducers to detect damages in HSS.