In this review we examine the use of secretion systems by bacteria to subvert host functions. Bacteria have evolved multiple systems to interact with and overcome their eukaryotic host and other prokaryotes. Secretion systems are required for the release of several effectors through the bacterial membrane(s) into the extracellular space or directly into the cytoplasm of the host. We review the secretion systems of Gram-positive and Gram-negative bacteria and describe briefly the structural composition of the seven secretion systems that have been associated with increased virulence through subversion of host functions. Some of the effects of such systems on eukaryotic host processes have been studied extensively. We also describe the best-characterized effectors of each secretion system to give an overview of the molecular mechanisms employed by bacteria to hide from the immune system and convert eukaryotic cells into optimal ecological niches for their replication.