High-quality observations of the Sun are widely available, gathered by NASA’s space-based flagship heliophysics mission, the Solar Dynamics Observatory (SDO) and by the continuous ground-based monitoring network, GONG. These exquisite observations reveal tantalizingly complex physics at the surface and in the atmosphere, the layers of the Sun that are optically accessible. Whereas, our understanding of processes in the solar interior still remains incomplete. Because the interior cannot be optically imaged, we rely on measuring and interpreting signatures of acoustic waves that propagate within and emerge at the surface. The physics of solar oscillations is well understood and seismic observations can be accurately explained using the theory of linear wave propagation in stratified media. Indeed linear wave physics has been shown to be successful at predicting oscillation frequencies to within parts of a thousand (Christensen-Dalsgaard 2002). This accuracy has allowed for the trustworthy inference of internal differential rotation and structural properties such as the sound speed and composition using measurements of solar oscillation frequencies. That these results have been reproduced by groups around the world (e.g., see, Christensen-Dalsgaard 2002; for a review) using a range of inverse methodologies has strengthened the significance of the inferences. As a consequence, helioseismology has matured into a precision science.