With the increasing global demand for electric vehicles and large-scale energy storage systems, solid-state batteries (SSBs) have emerged as a promising alternative to conventional lithium-ion batteries due to their enhanced safety and energy density. The rigid nature of solid-state electrolytes (SSEs) allows SSBs to avoid flammable liquid electrolytes and minimize the risk of thermal runaway. However, the complexity of solid–solid interfaces, the growth of lithium dendrites, and poor interfacial contact during battery operation pose significant challenges to their performance and longevity. Understanding these internal phenomena in real time thus becomes a critical research priority.Ultrasound and optical fiber sensing technologies are increasingly recognized as effective tools for in-situ, non-destructive, and real-time monitoring of internal battery behaviors. Compared with conventional electrochemical and spectroscopic methods, these techniques provide unique advantages in terms of spatial resolution, penetration depth, and multiparameter detection. This review outlines the fundamental principles, technological advancements, and practical applications of both ultrasound and optical fiber sensors in the context of solid-state batteries.Ultrasound technology utilizes high-frequency mechanical waves to analyze internal structural changes. Some parameters such as acoustic impedance, attenuation, and time-of-flight (ToF) provide valuable insights into material density, elastic modulus, and crack formation. These features allow ultrasound to dynamically evaluate gas evolution, pore formation, and interfacial degradation within batteries. For instance, customized ultrasonic imaging systems are developed to detect gas generation rates and interfacial contact loss in SSEs, enabling accurate assessments of degradation mechanisms. Research from our group indicates that ultrasound can effectively distinguish between chemical passivation and physical delamination at the electrode–electrolyte interface and track the impact of polymer cross-linking on interface uniformity during in-situ polymerization.Optical fiber sensing, especially fiber Bragg grating (FBG) technology, offers a high sensitivity to strain and temperature, making it well-suited for harsh battery environments. Embedded within cells, FBG sensors enable real-time monitoring of stress evolution and thermal distribution across electrodes and interfaces. Their immunity to electromagnetic interference, compact form factor, and low cost enhance their applicability in sealed battery systems. FBG sensors can effectively decouple strain and temperature responses via analyzing wavelength shifts induced by mechanical or thermal stimuli. Advanced designs, such as micro-FBGs with reduced diameters, further improve integration with battery components, minimizing interference with electrochemical performance. Beyond mechanical and interfacial diagnostics, these sensing technologies also allow for monitoring of key physicochemical parameters like internal temperature gradients, refractive index changes, and byproduct formation through distributed optical networks and acoustic mapping. Recent developments on lab-on-fiber platforms, which integrate Raman scattering and near-infrared spectroscopy with structural sensing, offer a multimodal approach for revealing degradation mechanisms and supporting real-time diagnostics in solid-state batteries.As emerging in-situ monitoring technologies, ultrasound and optical fiber sensing have significant advantages over conventional battery characterization methods. Ultrasound provides a non-destructive, efficient, and cost-effective means to sensitively detect internal pore evolution, gas generation, and electrolyte wetting, addressing limitations in penetration depth and response speed of conventional techniques. Optical fiber sensing enables real-time monitoring of structural phase transitions, ion migration, and stress distribution within batteries, with intense electromagnetic immunity, low cost, and flexible integration-particularly suited for long-term in-situ monitoring of sealed battery systems. While many reviews focus on the application of these techniques in lithium-ion batteries, their use in solid-state batteries remains relatively underexplored. This review systematically represents recent development and application of ultrasound and optical fiber sensing technologies to solid-state batteries from three perspectives, i.e., interfacial behavior, mechanical properties, and physicochemical characteristics. It also summarizes relevant work conducted by our research team and discusses future integration with intelligent battery systems, highlighting key opportunities and remaining challenges.Summary and prospectsUltrasound and optical fiber sensing technologies are poised to play a transformative role in the next generation of battery diagnostics. Their unique ability to perform high-resolution, real-time, and multi-physics monitoring of internal battery phenomena represents a significant advancement over conventional diagnostic approaches. These sensing techniques offer some possibilities for tracking complex degradation mechanisms and enabling proactive battery management strategies. Despite the notable progress achieved in recent years, several critical technical challenges persist. First, signal decoupling in complex environments, in which multiple physical processes such as gas evolution, crack propagation, and stress accumulation occur simultaneously, remains a considerable obstacle. Distinguishing these overlapping signals requires advanced data processing and modeling techniques. Second, the development on integrated, multi-sensor platforms that combine ultrasound, fiber–optic, and electrochemical sensors is still at a nascent stage. Such hybrid systems have a potential to provide more comprehensive and cross-validated data but require further innovation in sensor compatibility and system integration. Third, translating lab-scale sensing systems into practical applications for large-format battery packs demands further progress in miniaturization, cost-effectiveness, and long-term operational stability under dynamic conditions. Future research efforts should focus on three pivotal areas. At the interface level, there is a pressing need to develop multi-parameter sensing systems capable of simultaneously capturing interfacial gas release, contact resistance changes, and local stress distribution. At the mechanical level, high-resolution ultrasonic imaging combined with distributed optical fiber networks can enable real-time, spatially resolved tracking of crack initiation and propagation from micro-scale to macro-scale. At the physicochemical level, integrating spectroscopic methods such as Raman or near-infrared spectroscopy-with ultrasound and optical fiber sensing can unlock deeper insights into phase transitions, compositional changes, and side reactions. Solid-state batteries can achieve real-time diagnostics and early fault detection via utilizing advanced sensing data intelligence and material innovations, accelerating their path to reliable commercial deployment.