###### Abstract  
The propagation of ultrasonic guided waves in solids is an important area of scientific inquiry due primarily to their practical applications for the nondestructive characterization of materials, such as nondestructive inspection, quality assurance testing, structural health monitoring, and for achieving material state awareness. This Special Issue of the journal covers all aspects of ultrasonic guided waves (e.g., phased array transducers, meta-materials to control wave propagation characteristics, scattering, attenuation, and signal processing techniques) from the perspective of modeling, simulation, laboratory experiments, or field testing. In order to fully utilize ultrasonic guided waves for these applications, it is necessary to have a firm grasp of their requisite characteristics, which include being multimodal, dispersive, and comprised of unique displacement profiles through the thickness of the waveguide. The majority of the manuscripts in this Special Issue report on waveguides that are pipes and plates, but rods, rail, multi-strand wire, human bone, and half-spaces are also investigated. [PERSON] et al. [1] provide experimental results on an aluminum plate, demonstrating how wave propagation can be manipulated by changing the boundary conditions. Specifically, surface-mounted resonators block the propagation of the S0 Lamb mode at low frequencies. Forbidding the propagation of the S0 Lamb mode turns out to be very different from forbidding the propagation of the A0 Lamb mode but can be explained in terms of the boundary conditions, which provide an alternative approach to meta-materials. The remaining 18 manuscripts pertain directly to the detection of defects within the waveguide in order to enable the assessment of the structural integrity. While the purpose of pipes is to transport or circulate fluids, their hollow cylindrical geometry forms an efficient waveguide for longitudinal, torsional, and flexural ultrasonic waves. Torsional waves are often preferred for long-range defect detection because the wave energy does not leak into the fluids inside the pipe. Six manuscripts in the Special Issue investigate ultrasonic guided waves in pipes for the purpose of defect detection. [PERSON] et al. [2] show that an array of fiber Bragg grating (FBG) sensors multiplexed along an optical fiber can receive torsional guided waves in a pipe, thus improving the range of piezoelectric and magnetostrictive actuators because reflections do not have to propagate back to the actuator. [PERSON] et al. [3] demonstrate the use of split-spectrum processing to improve the signal-to-noise ratio (SNR) of axisymmetric torsional waves in pipes and investigate its limitations. They show a 30 dB increase in SNR. [PERSON] et al. [4] first propose a method to detect defects in pipes from axisymmetric torsional waves having a low signal-to-noise ratio by leveraging the signals received from individual elements in the ring array transducer. Then, they [5] propose an adaptive leaky normalized least-mean-square filter to reduce the effect of unwanted flexural waves on the detection of axisymmetric waves. In a third paper [6], the same authors describe the exploitation of the power spectrum to distinguish torsional waves from flexural waves in pipes for more reliable defect detection. Taking a different approach, [PERSON] and [PERSON] [7] propose applying a multilayer perceptron neural network to detect leaks in pressurized natural gas pipelines, based upon acoustic emissions from the leaks. Accurate localization depends on the group velocity of the flexural waves generated during the acoustic emission events. The Lamb waves and shear-horizontal (SH) waves that propagate in traction-free homogeneous isotropic plates are often used for defect detection when access is limited or to cover a large area. [PERSON] and [PERSON] [8] demonstrate how the analysis of partial waves provides new perspectives on the propagation characteristics of elastodynamic guided waves in plates as well as other types of waveguides. [PERSON] et al.