Pseudomonas aeruginosa is an opportunistic bacterial pathogen, capable of causing both acute and chronic infections in humans. The establishment of infection is a result of the broad pathogen-host interactions. P. aeruginosa possesses a variety of virulence factors to gain advantage over the host during such interactions, and among them are the complex type three (T3SS) and type six (T6SS) secretion systems, which help the pathogen to invade, colonize and survive in the host environment. Both T3SS and T6SS play important roles in cell-cell interaction and pathogenesis. Expression and function of these systems relies on various extracellular signals, such as calcium levels, phosphate concentrations and the contact with host cell. Much is still unknown about the complex regulation of the two secretion systems. A transposon insertion library was previously constructed in our lab to identify genes involved in T3SS regulation. In this project, the gene fimV and a gene cluster PA3284-81 identified from the library were selected for further study. Their effect on T3SS expression was verified and the potential mechanism investigated. The results showed that fimV is important for P. aeruginosa virulence and persistence, and the knockout fimV mutant downregulated the expression of exoS, a representative of T3SS system. PA3284-81 also influenced T3SS expression, and a significant increase of ExoS protein secretion was observed when PA3284-81 was over-expressed on a plasmid. Finally, T3SS expression was found to be regulated by zinc ions; an upregulation of exoS was observed under conditions of zinc depletion. In addition, PA3284-81 was shown to respond to other environmental signals, such as calcium, iron and peracetic acid, possibly influencing virulence. To investigate the regulation of the recently discovered T6SS systems, a transposon mutagenesis library was constructed. Potential regulators of the second T6SS system H2-T6SS were screened for. A Total of eight putative regulator genes were identified, in which two genes, one that downregulated (norC), and the other that upregulated (PA0961) H2-T6SS were investigated further. Significant decrease in H2-T6SS promoter activity as well as exoS promoter activity was observed in norC mutant, suggesting norC is a positive regulator of both H2-T6SS and T3SS. It is possible that the norC identified in the H2-T6SS transposon library serves as a link between T6SS and T3SS. In summary, this study has confirmed the involvement of fimV and PA3284-81 in the regulation of T3SS. The results also revealed that zinc condition affected T3SS expression, independently of PA3284-81. New factors including norC in T6SS regulation were also identified. These results provide a basis for further studies on the complex secretion systems in P. aeruginosa.