The Tat protein-export system serves to translocate folded proteins, often containing redox cofactors, across the bacterial inner membrane. Substrate proteins are directed to the Tat apparatus by distinctive N-terminal signal peptides containing a consensus SRRxFLK "twin-arginine" motif. Here we review recent studies of the Tat system with particular emphasis on the assembly of membrane-bound respiratory complexes. We discuss the connection between Tat targeting and topological organisation of the complexes and consider the role of chaperone proteins in cofactor insertion and Tat targeting. The crystal structure of Escherichia coli formate dehydrogenase-N demonstrates that some Tat substrates are integral membrane proteins. Sequence analysis suggests that one-quarter of all traffic on the E. coli Tat pathway is inner-membrane proteins.