• Provides an overview of the principles, capabilities, advantages, and limitations of various advanced in situ characterization techniques.
• Reviews in situ studies of fuel cells, water electrolysis, CO2 reduction reaction, and lithium batteries across multiple scales, from materials to surroundings.
• Highlights dynamic tracking of chemical and structural evolution of overall reaction systems, including materials, intermediates, products, and surroundings during operation.
• Proposes future integration of multimodal in situ/operando approaches with artificial intelligence for real-time monitoring at practical scales.