Frontiers of solid-state batteries. This issue of MRS Bulletin focuses on the current state of the art of solid-state batteries, especially electrolytes and device architectures, and provides perspective on the various materials and interfacial challenges that limit performance and stability. The cover shows a schematic of a cathode composite. The use of mechanical milling to form finer submicrometer particles allows for a larger contact area between a solid electrolyte, active sulfur, and conductive carbon. This creates effective ion- and electron-transport pathways. The green circles are sulfur; the gray circles are solid electrolytes; and the black circles are carbon. The red arrows show the electron transport, while the blue arrows show the Li-ion transport. Credit: Hyung Lim, Kangwon National University. See the technical theme that begins on page 740.