Protein secretion is a universal process of fundamental importance for various aspects of cell physiology including the infection of a host organism by a bacterial pathogen. Many Gram-negative pathogens export virulence-associated proteins across one or two cell membranes to their place of action using a wide plethora of secretory pathways with the objective of infecting the host. For Legionella pneumophila, a facultative intracellular, human pathogen which is ubiquitously found in natural and artificial aquatic environments, two major secretory pathways known to be involved in virulence have been described. These are the PilD-dependent Lsp type II secretion pathway and the type IV secretion system encoded by the dot/icm genes. In addition, a second type IV system, with high sequence similarity to the Agrobacterium tumefaciens VirB system for conjugal transfer of oncogenic DNA, is present. Albeit dispensable for intracellular growth, this type IV system is important for efficient host cell infection at lower temperatures. Further more, evidence exists for the presence of at least one type I secretion system in L. pneumophila as well as for the presence of a twin arginine dependent translocation (Tat) pathway. This is a recently detected, signal peptide-dependent, secretion pathway complementary to the well-known Sec-dependent pathway for protein transport across the cytoplasmic membrane.