Microbial cell surfaces, surface-exposed organelles, and secreted proteins are important for the interaction with the environment, including adhesion to hosts, protection against host defense mechanisms, nutrient acquisition, and intermicrobial competition. Here, we describe the structures of the cell envelopes of bacteria , fungi, and oomycetes , and the mechanisms they have evolved for the transport of proteins across these envelopes to the cell surface and into the extracellular milieu