Living organisms are open systems in constant and selective exchange of matter, energy, and information with their surroundings. In diderm bacteria, the presence of a dual-membrane cell envelope imposes a topological barrier for the trafficking Membrane traffickingof high molecular weight macromolecules. To allow the controlled transport of macromolecules such as proteins and nucleic acids across their protective cell envelope, diderm bacteria have evolved a series of sophisticated transmembrane appendages that enable the selective movement of these large polymers beyond the cytosol. Because these appendages function as physical interfaces between the intracellular compartment and extracellular environment, they are commonly integrated within and span the double lipid bilayer of the bacterial envelope. Remarkably, some of these appendages possess the capacity to traverse not only the bacterial membranes but also a third additional host cell membrane, allowing the direct delivery of macromolecules into eukaryotic or prokaryotic target cells. This chapter provides an overview of the overall architecture and function of transmembrane appendages in diderm bacteria, with particular emphasis on bacterial secretion systems that transport proteins across the cellular envelope in one single step.