Ultrasound application for Structural Health Monitoring (SHM) is based on permanently installed sensors. Signals received after either active excitation or acoustic emission (AE) caused by structure load are used to locate the position of possible defects. Ultrasonic guided waves (GWs) are of particular attention because these waves can travel long distances with low attenuation. However, defect location using direction of arrival (DoA) is tampered by angle estimation error dependence on the arrival angle. [PERSON] et al. [4] proposed a novel approach in sensor cluster design: a specially shaped cluster instead of a conventional triangle formed by three round sensors. The sensor shape optimization and direction estimation procedure was based on Radon transform. The proposed shaping of the piezo-patches ensures that the difference in time of arrival between reference sensor P1 and sensor P2 is a linear with angle, whereas the difference between P1 and P3 is constant across the considered range of DoA. In such case, the uncertainty of DoA estimation is improved. Furthermore, DoA can be estimated without knowing the actual wave mode velocity. SMH can benefit from fact that the speed of GW is proportional to the stress level of the medium. This can be used to monitor the strain level of the structure. [PERSON] et al [5] proposed the sensitivity to strain enhancement by a post-processing procedure which preserves only the strain-sensitive spectral components. The proposed technique was demonstrated on the aluminum plate, obtaining up to twice higher sensitivity to strain. Diagnostic imaging in medicine using ultrasound is offering several benefits: there is no exposure the ionizing radiation, and acoustic waves directly interact with tissue mechanical properties. Not just contrast, but elasticity of the possible abnormalities can be investigated. However, tissue signal attenuation with distance finally makes electronic noise predominant with respect to tissue speckle. The maximum depth of penetration (DoP) is used as an indicator of how deep the imaging system is able to display the anatomical features. DoP depends on the equipment, signal and processing type used. Therefore, DoP has to be evaluated in order to make sure that clinician can identify and characterize possible abnormalities. Currently DoP is evaluated manually. [PERSON] et al. [6] proposed an automated procedure an adaptive Signal to Noise Ratio threshold method (AdSTM) and compared it to an alternative technique, tangent threshold method (TTM). Results indicate that AdSTM have a higher sensitivity than the TTM. For a scanner with limited gray level dynamic range, the TTM was less reliable. AdSTM require less computing therefore resulted in twice lower processing time. Submissions for this Special Issue have been closed, therefore just a small fraction of the topic was covered. Nevertheless, it can be concluded that transduction is still limiting the ultrasonic systems performance. That is why new piezoelectric materials, sensitivity, bandwidth and durability of ultrasonic transducers are under constant development. Yet, electronics and signal processing are the counterparts that should not be forgotten, since only mutual optimization can ensure full system performance. Fortunately, new developments in these areas continue to emerge, offering new ultrasound applications.  
Guest Editor would like to thank the authors who contributed to this Special Issue 'Ultrasonic Transducers and Related Apparatus and Applications'. Special thanks to the reviewers, who dedicated their time to ensure high scientific standard of publications. The author declares no conflict of interest.