Solid-state batteries promise higher energy density and safety, but simply replacing liquid electrolytes with solid ones does not guarantee improvement over lithium-ion batteries. Achieving higher energy density requires high active material (AM) content and high AM loading, which is hindered by solid-state electrolyte’s (SSE’s) low ionic conductivity, poor AM–SSE interfaces, sluggish Li$^+$ transport, and processing challenges. In this review paper, the fundamental mechanisms of ion transport in SSEs and composite electrodes are comprehensively reviewed and discussed. It is found that reducing the internal ionic resistance and the diffusion impedance of AM are effective ways to boost the effective current density of the composite electrode and decrease the overpotential of SSBs to enhance the delivered energy. The mechanisms and advanced techniques for measuring both ionic and electronic conductivities, as well as directly observing the ionic conductivity and diffusion of the composite electrodes are summarized. Furthermore, the strategies for improving ion diffusion within the AM, and enhancing ion transfer across the high AM content composite electrode are discussed. In addition, the challenges associated with industrialization of composite electrodes and potential solutions are discussed. This review paper summarizes the key aspects of ion transport in the solid-state composite electrode, aiming to support the design of ultrahigh energy density SSBs.