This report provides an overview of diverse transit bus applications of advanced Lithium Ion Batteries (LIBs). The report highlights and illustrates several FTA programs that fostered the successful development, demonstration, and deployment of fuel-efficient hybrid-electric and electric drive transit buses in operational urban fleets over the last decade. The focus is on recent progress in the rechargeable energy storage systems (RESS) that successfully integrated the lighter, more compact LIBs with higher energy density and capacity in a broad range of power and propulsion configurations for urban transit bus fleets. Improvements in fuel efficiency and environmental performance of succeeding generations as well as LIB-related safety, cost, reliability, availability, and maintainability challenges are discussed in context, including recent recalls due to LIB safety issues. Progress in and prospects for future LIB improvements and remaining bus application challenges are also discussed.