Online acoustic emission (AE) sensing is a nondestructive method that has the potential to be an indicator of battery health and performance. Rechargeable batteries exhibit complex mechano-electrochemical behaviors during operation, such as electrode expansion/contraction, phase transition, gas evolution, film formation, and crack propagation. These events emit transient elastic waves, which may be detected by a piezoelectric-based sensor attached to the battery cell casing. Research in this field is active and new findings are generated continuously, highlighting its potential and importance of further research and development. This Review provides a comprehensive analysis of AE sensing in rechargeable batteries, aiming to describe the underlying mechanisms and potential applications in battery monitoring and diagnostics. This Review summarizes recent progress and discusses future perspectives in applying online acoustic emission sensing as a non-destructive method for monitoring rechargeable batteries.