The HtrA family refers to a group of related oligomeric serine proteases that combine a trypsin-like protease domain with at least one PDZ interaction domain. Mammals encode four HtrA proteases, named HtrA1-4. The protease activity of the HtrA member HtrA2/Omi is required for mitochondrial homeostasis in mice and humans and inactivating mutations associated with neurodegenerative disorders such as Parkinson's disease. Moreover, HtrA2/Omi is released in the cytosol, where it contributes to apoptosis through both caspase-dependent and -independent pathways. Here, we review the current knowledge of HtrA2/Omi biology and discuss the signaling pathways that underlie its mitochondrial and apoptotic functions from an evolutionary perspective.