This book explains theoretical, numerical, and experimental procedures for the structural health monitoring (SHM) of advanced sandwich composites, such as honeycomb composite sandwich structure. These structures are extensively used in aerospace, aeronautics, marine and automotive industries. But, these structures are prone to disbonds at the skin-core interphase, and the high-density core (HD-core) sections are often introduced inside the soft-core in order to provide localized support. It is essential to clearly identify those unknown HD-core regions and hidden disbonds in such kind of structures. The in-situ inspection methods are considered here, which do not require dismantling of the larger structure. This book mainly focuses on the development of ultrasonic guided wave propagation based active SHM frameworks to identify such hidden disbonds and HD-core regions in sandwich structures using piezoelectric transducers.