The properties of acoustic waves, such as their speed and attenuation, are influenced by the mechanical properties and geometry of the medium they traverse. In the case of lithium-ion batteries, this manifests as a correlation to the state-of-charge (SoC), since the mechanical properties and geometry of electrodes vary and evolve with the state. Demonstrations of this behaviour were made definitively in the past decade and were followed by attempts to leverage the correlation in constructing state estimators reliant on acoustic signals. The aim was never to replace conventional methods of SoC estimation, which can be highly successful (such as electrochemical-model-based approaches ), but rather to complement and diversify them, targeting improvements in safety and reliability. The potential for such improvements is based on the independence of the acoustic signal, which is measured by a dedicated sensor and is also uniquely informed by the internal structure and chemomechanics of a cell, rather than its aggregate electrochemical characteristics. Therefore, acoustic state estimation can be performed separately from other methods, and can be used to verify them, detect faults, or contribute towards the quantification of uncertainty.