Flexible lithium-ion batteries (LIBs) are considered the most promising energy storage solution for powering flexible electronics due to their versatility. However, significant advancements in functional materials, flexible components, and battery design are necessary to achieve high performance and stability under mechanical strain. While progress has been made in electrode, electrolyte, and other component materials, full cell structure design, assembly processes, and fabrication methods for flexible LIBs, several challenges remain. These challenges include low gravimetric and volumetric energy densities, limited capacity retention and cycle life after bending, uncertain performance at high rates and extreme temperatures, difficulties in automated production, and high fabrication costs. This chapter provides a critical review on the current development of flexible lithium-ion batteries for flexible electronic devices, including innovative designs of cell configuration for flexible and stretchable LIBs, selection of functional materials, and evaluation methods for flexible LIBs, as well as grand challenges involving in energy density and scale-up fabrication, and perspectives for future research and development directions.