Abstract: The global transition to sustainable energy systems requires breakthroughs in electrochemical storage technologies that are not only safe, but also resource-efficient. Solid-state batteries (SSBs), which use superionic solid electrolytes (SEs) instead of flammable liquid electrolytes, are at the forefront of this transformation. In general, SEs promise increased safety, access to high-voltage cathode and metal anode chemistries, and new avenues for circular design and recyclability. However, to reach their full potential, intertwined challenges related to ion transport, (electro)chemical stability, manufacturing, processing, and cost must be overcome. This “2026 Roadmap on Next-Generation Solid Electrolytes for Battery Applications” outlines new directions that will contribute to research in the field of SSBs over the next decade. It provides an overview of the current state of the art in sulfide- and halide-based solid electrolytes for Li and Na systems, examines post-Li/Na chemistries (K, Mg, and others), and highlights advances in hydroborates, fully reduced (irreducible), and compositionally complex (high-entropy) electrolytes, as well as glass-ceramic electrolytes. Beyond material innovation, the paper emphasizes the critical role of redox activity in SEs, scalable processing, high-throughput synthesis, and machine learning, as well as operando analytics and nuclear magnetic resonance spectroscopy to accelerate discoveries and gain a better understanding of structure-property relationships. Finally, the growing importance of recycling and circular design for ensuring sustainability is highlighted. By combining insights from chemistry, materials science, data (computational) science, and manufacturing, this article assumes that future SEs will progressively evolve from passive components to active design elements in high-energy-density electrochemical systems. The integration of multidisciplinary innovations will be crucial to realizing the potential of SSBs in practical technologies that power a decarbonized world.