The direct transport of virulence proteins from bacterium to host has emerged as a common strategy employed by Gram-negative pathogens to establish infections. Specialized secretion systems function to facilitate this process. The delivery of 'effector' proteins by these secretion systems is currently confined to two functionally similar but mechanistically distinct pathways, termed type III and type IV secretion. The type III secretion pathway is ancestrally related to the multiprotein complexes that assemble flagella, whereas the type IV mechanism probably emerged from the protein complexes that support conjugal transfer of DNA. Although both pathways serve to transport proteins from the bacterium to host, the recognition of the effector protein substrates and the secretion information contained in these proteins appear highly distinct. Here, we review the mechanisms involved in the selection of substrates by each of these transport systems and secretion signal information required for substrate transport.