Mitochondria are essential constituents of eukaryotic cells, and their numerous functions include cellular respiration and generation of ATP. To maintain these mitochondrial func­tions, it is crucial to secure protein homeosta­sis, i.e. maintenance of a functional protein population. This task is executed through a sophisti­cated protein quality control network of molecular chaperones and proteases. In par­ticular, the generation of reactive oxygen species at respiratory chain complexes is a major threat to the integrity of the organelle. This requires the removal of damaged pro­teins by the Pim1/Lon protease. As compo­nents of the protein quality control machinery are encoded by nuclear genes, mitochondrial stress conditions need to be signaled to the nucleus, a process that requires the action of another mitochondrial protease, ClpP.