Explore the theory and experiments behind mode-resolved phonon scattering spectroscopy in this 35-minute conference talk. Delve into the fundamentals of heat conduction as a non-equilibrium transport process, understanding phonons as heat carriers in semiconductors. Learn about first principles approaches to calculating thermal conductivity and the challenges of diffusive heat conduction. Discover solutions through non-diffusive heat conduction and quasiballistic heat conduction in real materials. Examine techniques like transient grating for optically changing the characteristic length and surface scattering of phonons in thin films. Gain insights into grating period-dependent thermal conductivity and the first spectral measurement of phonon-surface scattering, including its sensitivity to atomic-scale roughness.