Abstract: Phononic crystals (PC) are artificially engineered periodic structures that can alter the propagation characteristics of elastic/acoustic waves in ways that are not possible in conventional materials. Phononic crystal structures has recently attracted a growing research interest due to their unprecedented properties and functionalities such as Bragg band gaps and negative refraction, and also their potential for elastoacoustic wave devices in everyday life. However, due to the due to the scaling of the PC unit cell dimensions with the wavelength, applicability of PC structures is generally limited to the high frequency applications (~tens/hundreds of kHz). On the other hand, metasurfaces are formed by a single array of subwavelength-scaled unit cells that can offer new ways to modulate the elastic wave front for desired wave functionality with the smallest possible structure which is essential in low frequency applications (~hundreds of Hz). In this talk, I will present our most recent research on phononic crystals and metasurfaces that deal with guiding and manipulation of elastic waves toward different applications. The first part of my talk will focus on gradient index phononic crystal (GRIN-PC) lenses conforming pipe-like structures for improved sensing in non-destructive testing. GRIN-PC lenses are designed by tailoring unit cell geometry according to a special refractive index profile. Here, we explore focusing of multi-mode guided waves at the desired locations (i.e. sensor nodes) along the pipe structure to remedy the attenuation problem in the long-range pipelines. Then, I will explain how we exploit the negative refraction property of phononic crystals for designing a super lens. As an alternative to the GRIN-PC lenses which have at least minimum wavelength resolution as their natural limit, negative refraction-based PC lenses can potentially overcome the diffraction limit that is highly favorable in medical imaging or other applications that require localized wave intensity in an area smaller than a square wavelength. The second part of my talk will deal with the elastic metasurfaces for full wavefront control with an emphasis on energy harvesting of low frequency elastic waves. Here, we fully analyze and design the elastic metasurfaces by tailoring the phase gradient of the individual unit structures for different wave functions (i.e. deflecting, self-bending, and focusing). I will present our analytical, numerical, and experimental findings and demonstrate how we achieve nine-fold increase in the harvested power via the focusing metasurface.