Unlike their rigid counterparts, flexible batteries require every component—from the container to the electrodes and electrolyte—to be bendable and adaptable. This versatility opens up possibilities for diverse shapes and fabrication methods. Designing high-performing flexible batteries hinges on three key aspects: materials selection, structure engineering, and configuration construction. Advancements in high-energy active materials, multifunctional auxiliary components (current collectors, separators, electrolytes, packaging), and optimized configurations (planar and fiber structures) contribute significantly to improving electrochemical performance, mechanical stability, cost, and safety. This chapter delves into how flexible components, particularly inactive materials, influence both mechanical properties and battery performance. It also explores popular flexible configurations, including planar and fiber structures, explaining their functions and remaining challenges.