Abstract <p>Biocalorimetry offers a powerful approach for real-time process monitoring and optimization in the biotechnological utilization of both natural and synthetic macromolecules, particularly in complex solid-state systems aiming at the valorization of plant biomass or plastics’ waste. This review critically examines general strengths and limitations of biothermodynamics and calorimetry for monitoring microbial activity, which can be tracked across diverse scales and substrates via metabolic heat measurements. Metabolic heat-derived activity parameters enable the robust quantitative evaluation of the performance of microorganisms for substrate conversion, hence potentially representing valuable tools for bioprocess development and operation. While biocalorimetry is established in liquid-phase cultivation systems to some extent, its adaptation to solid-state fermentation, composting, and the biochemical breakdown of solid plastics still remains in early stages while holding promise for real-time control and upscaling. Challenges and limits of the applicability of this technology currently persist especially for mixed cultures and non-sterile processes. Nevertheless, expanding metabolic heat-based datasets to microbial functional traits could advance ecological and industrial applications. Overall, biocalorimetry is positioned as a valuable tool for advancing circular bioeconomy strategies, though further validation and methodological development are needed for broader adoption in both research and industrial contexts.</p> Key points <p>• <i>Biocalorimetry reliably quantifies microbial activity on complex solid substrates</i>.</p> <p>• <i>Biocalorimetry can support plant biomass and plastic waste valorization in a circular bioeconomy</i>.</p> <p>• <i>Biocalorimetric monitoring is applicable from the laboratory to the technical scale</i>.</p>