The research and development of solid polymer electrolyte (SPE) began when Wright found ion conductivity in a PEO-alkaline metal ion complex in 1975. The conductivity then was 1×10−7 S cm−1 at room temperature. A lithium polymer battery has features such as flexibility in the shape of a cell design, leak proof of electrolyte, high safety, etc., but poses the challenge of how close its electrical performance can be made to that of a liquid electrolyte cell. Therefore, various efforts have so far been made to improve the ionic conductivity of the SPE. Recently, such efforts have also included the development of gelled SPE and porous SPE, especially in consideration of its practical application, in particular the use at low temperature. The ionic conductivity of such SPEs now reaches 1×10−3 S cm−1 at room temperature. This paper reviews the history and the present status of the research and development of SPE and the lithium polymer battery, and presents an outlook of the future research and development activities. The paper also introduces the history of the improvement of primary and lithium polymer secondary batteries using SPE at the Yuasa Corporation, with the performance and the applications of its present commercial products, and presents their future outlook.