Abstract: In the crucial area of sustainable energy storage, solid-state batteries (SSBs) with nonflammable solid electrolytes stand out due to their potential benefits of enhanced safety, energy density, and cycle life. However, the complexity within the composite cathode determines that fabricating an ideal electrode needs to link chemistry (atomic scale), materials (microscopic/mesoscopic scale), and electrode system (macroscopic scale). Therefore, understanding solid-state composite cathodes covering multiple scales is of vital importance for the development of practical SSBs. In this review, the challenges and basic knowledge of composite cathodes from the atomic scale to the macroscopic scale in SSBs are outlined with a special focus on the interfacial structure, charge transport, and mechanical degradation. Based on these dilemmas, emerging strategies to design a high-performance composite cathode and advanced characterization techniques are summarized. Moreover, future perspectives toward composite cathodes are discussed, aiming to facilitate the develop energy-dense SSBs.