All solid state batteries (ASSBs) are among the remarkable next generation energy storage technologies for a broad range of applications, including (implantable) medical devices, portable electronic devices, (hybrid) electric vehicles, and even large scale grid storage. All solid state thin film Li ion batteries (TFLIBs) with an extended cycle life, broad temperature operation range, and minimal self discharge rate are superior to bulk type ASSBs and have attracted considerable attention. Compared with conventional batteries, stacking dense thin films reduces the Li ion diffusion length, thereby improving the rate capability. It is vital to develop TFLIBs with higher energy density and stability. However, multiple challenges, such as interfacial instability, low volumetric energy density, and high manufacturing cost, still hinder the widespread application of TFLIBs. At present, many approaches, such as materials optimization and novel architecture design, have been explored to enhance the stability and energy density of TFLIBs. An overview of these discoveries and developments in TFLIBs is presented in this review, together with new insights into the intrinsic mechanisms of operation; this is of great value to the batteries research community and facilitates further improvements in batteries in the near future.