The mucosal surfaces of metazoan organisms provide niches for colonization by commensal microbes. However, these barrier surfaces also encounter pathogens. Therefore, sentinel immune cells must be capable of distinguishing between pathogenic and non-pathogenic organisms to tailor appropriate immune responses. Virulent microorganisms often uniquely possess mechanisms for accessing the host cell cytosol. Therefore, to detect pathogens, innate immune cells encode cytosolic receptors, which recognize conserved, pathogen-associated molecular patterns. Many mammalian cytosolic receptors activate the inflammasome, a multi-protein complex that activates the host enzyme caspase-1. Caspase-1 mediates IL-1 family cytokine release and a pro-inflammatory form of cell death, which are important for host defense. The canonical inflammasome activates caspase-1, but recent studies have shown that a related enzyme, caspase-11, contributes to inflammasome activation. However, it remains unclear if caspase-11 mediates inflammasome responses against bacteria that use virulence-associated secretion systems to deliver bacterial products into the host cytosol. Additionally, humans encode two orthologs of caspase-11, caspase-4 and caspase-5, and it is unclear if either enzyme contributes to inflammasome activation in primary macrophages. Furthermore, the bacterial ligands that trigger inflammasome activation in human cells are poorly defined. Legionella pneumophila, which causes pneumonia, uses a specialized secretion system to access the host cytosol to establish a replicative niche in both murine and human cells. Therefore, we investigated the host and bacterial requirements for inflammasome activation in response to L. pneumophila, and we interrogated if these requirements are conserved for the response against other Gram-negative bacterial pathogens. Our studies demonstrate that caspase-11 contributes to IL-1 release and cell death in response to bacterial pathogens in murine macrophages, and we find that inflammasome activation requires the presence of virulence-associated secretion systems. Using neutralizing antibodies, we show that IL-1α and IL-1β have distinct roles in pulmonary defense against L. pneumophila in vivo. Through siRNA knockdown studies, we demonstrate that human caspase-4 has a conserved role in inflammasome activation in response to multiple Gram-negative bacterial pathogens. Finally, using bacterial mutants, we show that flagellin is a trigger for inflammasome activation in human macrophages. Overall, our studies help define the mechanism by which host cells initiate defense against bacterial pathogens.