**Abstract**

Batteries are being seen as a key technology for battling CO2 emissions from the transport, power, and industry sectors. However, to reach the sustainability goals, they must exhibit ultrahigh performance beyond their capabilities today. So, it is becoming crucial to develop advanced diagnostic/prognostic tools injected into the battery that could nonintrusively track in time and space its physical and chemical parameters, for ensuring a greater lifetime and therefore lower its CO2 footprint. In this context, a smart battery sensing system with high performance and easy implementation is critically needed for the vital importance of safety and reliability in all batteries. Parameters like temperature (heat flow), strain, pressure, electrochemical events from electrode lithiation to gassing production, refractive index, and SoX battery indicators are of high importance to monitor. Recently, optical fiber sensors (OFS) have shown to be a feasible, accurate, and useful tool to perform this sensing, due to their intrinsic advantages and capabilities (lower invasiveness, multipoint and multiparameter detection, capability of multiplexing being embedded in harsh environments, and fast response). This chapter presents and discusses the studies published regarding the different types of OFS, which were developed to track several critical key parameters in Li-ion batteries, since the first study was reported in 2013.

**Keywords:** optical fiber sensors, smart sensing, *in situ* monitoring, Li-ion battery performance, safety
