As a monoderm prokaryote, protein secretion systems in Listeria monocytogenes are distinct from those encounter in diderm bacteria, still they remain the gates for expressing protein functions outside the intracellular bacterial cell compartment. Despite the fact that protein secretion is a key factor in virulence of a pathogen, fewer studies have been dedicated to pathogenic Gram-positive bacteria compared to Gram-negative bacteria and L. monocytogenes is no exception. Among the six protein secretion systems identified in L. monocytogenes, only proteins putatively translocated via the Sec pathway are indisputably involved in bacterial virulence. The 16 secreted virulence effectors characterized to date are either (i) associated with the cytoplasmic membrane, i.e. as integral membrane proteins or lipoproteins, (ii) associated with the cell wall, i.e. covalently in a sortase-dependent manner or via cell-wall binding domains, or (iii) released in the extracellular milieu. Identification of several candidates as putative secreted virulence factors as well as the availability in the near future of a large amount of Listeria genomic data from different sequencing projects promise a very exciting time in the field of listerial protein secretion and should provide further insights into how L. monocytogenes interacts with its biotic or abiotic surroundings. Introduction The genus Listeria is hitherto circumscribed to only six species namely L. monocytogenes, L. in-nocua, L. seeligeri, L. welshimeri, L. ivanovii and L. grayi (Vaneechoutte et al., 1998); it is worth