Thermophilic anaerobic processes are often regarded as less stable than mesophilic processes. In the paper this postulate is examined and disproved based on real operational data from of full-scale mesophilic and thermophilic biogas plants. The start-up procedure for the thermophilic plants was, however, found to be crucial for determining the period needed before full capacity of the thermophilic plant could be achieved. The importance of balanced loading, determined by the concentration of active biomass, is discussed. Finally, data will be shown on the optimal and maximal temperature for thermophilic digestion along with the implications for the methanogenic bacteria active at these temperatures.