Toyota introduced the first generation Prius in 1997. The vehicle was conceived, designed and launched as a dedicated, mass-produced global hybrid vehicle platform, the first of its kind. The introduction of the 2nd and 3rd generation Prius (2003, 2009) saw vehicles with significantly improved performance, including fuel efficiency. The Prius Alpha (Japan/EU), launched in 2011, represented Toyota first foray with Li-ion battery in a strong hybrid configuration. For the Prius Alpha, the adoption of a compact Li-ion battery resulted in sufficient cabin space to allow a 3rd row of seats while maintaining high fuel efficiency. Before and after the launch of the Prius Alpha, an extensive list of tests was performed on the Li-ion battery pack, including electrical, electrochemical, mechanical, and safety. The evaluations were performed in the lab, in the field (demonstration fleets) and by acquiring vehicles used by customers. The MY16 Prius (4th generation) which is launched in 2015, offers a Li-ion battery pack. The array of data available from the Prius Alpha evaluations, were considered in the development of the battery pack for the MY16 Prius. The state of the art battery in the MY16 Prius represents Toyota’s most improved Li-ion battery developed purposefully for strong hybrid traction application. This paper will discuss results from evaluations of Prius Alpha batteries recovered from the market and introduce the technical concept for the state of the art Li-ion battery technology for the MY16 Prius.