The performance and reliability of nanoscale devices are strongly influenced by the transport and dissipation of energy from hot charge carriers generated under optical or electrical excitation. In semiconductors, hot carriers relax their excess energy through scattering with phonons, after which energy dissipation proceeds via phonon transport. However, at the nanoscale, limited phonon–phonon interactions and insufficient thermalization can drive phonon populations out of equilibrium, resulting in distinct local temperatures for optical and acoustic phonon modes. This phonon non-equilibrium significantly affects heat dissipation efficiency and can degrade the thermal stability of nanoelectronic systems. This chapter focuses on Raman-based characterization of phonon non-equilibrium in various material systems. Section 8.1 introduces the experimental and theoretical challenges associated with probing non-equilibrium phonon populations. Section 8.2 describes Raman approaches for quantifying optical phonon temperatures. Section 8.3 discusses techniques for distinguishing optical and acoustic phonon temperatures in two-dimensional (2D) materials. Section 8.4 extends these methods to three-dimensional (3D) crystals. Section 8.5 explores phonon non-equilibrium at interfaces, where mismatched vibrational spectra hinder thermalization across material boundaries.