On the smaller scale, the propagation of guided waves has been modeled to help predict corrosion [25]. In contrast to theory and modeling are experimentalists who take a more hands on approach and perform testing. Examples include the flexural testing of high-performance steel beams, steel bar corrosion [26], testing of bonding behavior in reinforced concrete [27], among others [28; 29]. At a higher level, using the current understanding of system mechanical behaviors, investigators have developed methods to identify, extract features, and classify the health of different structures using minimal knowledge of system behavior [30; 31; 32; 33; 34; 35]. Supporting such work are other researchers that specialize in the long-term monitoring of damage and help strategize everything from sensor placement to understanding what the key parameters that need to be monitored are. Examples include work on developing of positioning systems that determine the best locations to measure structural vibrations, monitoring of tunnel deformations using GPS systems, monitoring of pipes to blast forces, etc. [36; 37; 38; 39; 40; 41]. A large number of investigators focus on damage detection [42; 43; 44; 45; 46], in which sensing technologies are used to characterize (e.g., location, size, depth, etc.) damage at a shorter time frame than typical monitoring techniques. Examples include the use of vibration and acoustic signals to detect water leaks [47], the use of infrared to detect damage in beams [48], utilizing acoustic emission to detect wearing in pin connections [49], and the training of deep learning neural networks to identify damage in composite pipelines [50], and much more [51]. An interesting subset of the research on SHIM and damage detection is driven by the multi-functionalities of piezoelectric materials [52; 53; 54], such as PZT (Lead Zirconate Titanate), which has a strong piezoelectric effect and is often used to generate and detect stress waves for SHM [55; 56; 57]. PZT has high bandwidth [58; 59] and is often used to fabricate acoustic emission sensors/probes [27; 49]. As reported in this special issue, PZT transducers have demonstrated the potential for a staggering range of applications [25; 26; 27; 27; 35; 42; 49], including the development of smart transducers that have recently shown to be able to monitor the health of structural components and interfaces and detect debonding and delamination damages. A recent development has even demonstrated the ability to use piezoelectric transducers to communicate information across a structure [60], potentially also picking up on damage as the stress waves propagate across the structure. All the above research and development have the potential to work with each other to produce new technologies and understanding in ways that cannot be expected. Examples can be found in the most recent research in bolt looseness detection [61]. The bolt is among the most ubiquitous structural component found in almost all types of infrastructure [62; 63]. The importance of the bolted connection is for instance supported by recent Bureau of Safety and Environmental Enforcement (BSEE) statements that warn how disastrous oil spills can result from neglecting to constantly inspect and monitor bolted connections in offshore equipment. Recent research has shown the innovative ways to use vision [62] and vibro-acoustic signals [63] to rapidly detect and even quantify bolt looseness. For example, by listening to the sounds generated by the light percussion of a bolt by a small impactor [64; 65], a neural network, combined with complex signal processing algorithms, can tell the operator the preload of the bolt [66], and potentially what other bolts (e.g., if installed in a flange) need servicing. The percussion-based method shall have potential in damage detection of various civil structures, such as concrete filled steel tube structures and steep plate composite structures.