Mitochondria are responsible for energy transduction and other critical metabolic processes in eukaryotic cells. To maintain function, mitochondria contain many levels of quality control. The first level is protein quality control, which is facilitated by a network of molecular chaperones and proteases, that assist in the proper folding of newly imported proteins, the refolding of misfolded proteins and the removal of irreversibly damaged or unassembled proteins. Our studies focus on two of the major AAA+ proteases in human mitochondria, LONM and CLPXP. To date neither the substrates nor mechanism by which substrates are recognised by these proteases is well understood. Research into the substrate specificity of these two AAA+ proteases will be reported.