Mechanical spectroscopy investigates materials behavior using acoustic techniques. This technique saw its heyday in the 1960s and 1970s, but recent advances in measurement, modeling, and characterization open up new possibilities in this field. Novel techniques such as resonant ultrasound spectroscopy (RUS) and atomic force microscopy (AFM)-based local acoustic measurements enable high-precision investigations of previously inaccessible phenomena. Atomistic and mesoscale modeling, high-resolution microscopy, and modern microstructure characterization methods offer new approaches for interpreting acoustic data. Judicious integration of measurement, modeling, and characterization may transform classic mechanical spectroscopy into a powerful tool for investigating the micromechanisms of materials behavior. The international symposium ‘‘New Horizons for Mechanical Spectroscopy in Materials Science’’ was held at the TMS 2015 Annual Meeting and Exhibition with the aim of addressing emerging opportunities for applications of mechanical spectroscopy in materials science. Emphasis was given to new mechanical spectroscopy techniques, integration of mechanical spectroscopy with materials modeling, and applications to complex solids. Several exciting recent developments were reported at this symposium. For instance, the notion of defects in crystalline structures is easy to grasp; but is it possible to define a defect in a material that has no long-range order such as a glass? This question has been explored numerically, using molecular dynamics, and experimentally. Whatever the exact nature of hypothesized defects in amorphous solids, they should interact with acoustic waves. Thus, mechanical spectroscopy provides a nonintrusive way of ascertaining their properties. In this topic, M. Atzmon presented results about the role of shear transformation zones on the dynamical response of glasses, and L. Huang explained the use of Raman and Brillouin scattering to understand the behavior of glasses under extreme conditions. Another topic was the use of ab initio numerical codes or multimillion atom classic potential simulations to explore the effect that defects have on macroscopic acoustic response of crystalline solids. E. Bitzek showed results on the dynamics of dislocations at high velocities using molecular dynamics, and E. Clouet used quantum mechanical codes to shed new light on the internal friction behavior of irradiated zirconium. A third example concerned the mechanical behavior of surfaces, thin films, and interfaces, all of which might be interrogated both through atomistic numerical simulation and elastic waves. This topic was highlighted by the talk of R. Schwarz, who showed how to use Rayleigh waves to study metal– metal interfaces. Resonant ultrasound spectroscopy, in which the normal modes of vibration of a material provide information about its microstructure, has evolved into an amazingly accurate technique that can explore many different phenomena through their coupling to the material strain. A. Migliori gave an overview of the current capacities of this technique, M. Hirao showed how it can be used to explore the local variation of the elastic constants of a material, and M. Carpenter provided results on a variety of materials behavior near a phase transition. In the related field of laser ultrasonics, D. Hurley provided measurements of local elastic properties of nuclear fuel surrogates. Michael Demkowicz is the guest editor for the Chemistry and Physics of Materials Committee, a joint committee of the TMS Functional Materials Division (FMD) and the TMS Structural Metals Division (SMD); and coordinator of the topic New Horizons in Mechanical Spectroscopy in this issue. JOM, Vol. 67, No. 8, 2015