The most simple membrane protein insertion catalyst known so far is the universal YidC/Oxa/Alb insertase that is composed of a single multi-spanning protein present in archaea, bacteria and in eukaryotic organelles. In bacteria, YidC is known to integrate small membrane proteins on its own and more complex proteins in conjunction with the SecYEG translocase. In mitochondria, the YidC homologue Oxa is responsible for the insertion of all membrane proteins synthesized in the matrix since no Sec homologues are present in the mitochondrial inner membrane. This is tantamount to the observation that YidC is able to operate also independently of SecYEG in bacteria. Reconstituted into liposomes, YidC rapidly and efficiently binds to substrate proteins and leads to their integration into the bilayer. Additionally, single molecule force spectroscopy experiments show that YidC binds to unfolded membrane proteins and promotes their folding into the membrane bilayer. To achieve membrane insertion and the correct folding, the periplasmic regions of the substrate have to cross the membrane with the help of YidC by a mechanism that is presently explored.