Identifying the observables that warn of volcanic unrest and eruptions is one of the greatest challenges in natural hazard management. An important observable is the release of heat through volcano surfaces, which represents a major energy source at quiescent volcanoes.  However, many questions remain open: Is there a direct correlation between surface heat emissions and the subsurface processes preceding volcanic eruptions? To what extent are volcanic eruptions preceded by an enhancement of the surface heat emissions? In this talk, I will address these questions using different volcanoes as case studies, including Nyiragongo (Congo) and Cumbre Vieja (La Palma, Spain), two of the most socially impactful volcanoes in 2021.