During Salmonella infection, bacterial proteins called ‘effectors’ are translocated into host cells by two type III secretion system apparatuses encoded by Salmonella-pathogenicity island 1 and 2. These effectors manipulate host cell processes to facilitate the formation of an intracellular replicative niche, to prevent bacterial clearance, and ultimately promote bacterial transmission to another susceptible host. A subset of SPI-2 T3SS effector proteins manipulate innate immune signalling pathways thereby preventing formation of an appropriate immune response. In this thesis, I identify three related effector proteins - GtgA, GogA, and PipA, as sufficient to inhibit NF-kappaB signalling when expressed ectopically. Furthermore, I demonstrate that GtgA, GogA, and PipA are zinc metalloproteases that inhibit NF-kappaB signalling by cleaving the NF-kappaB transcription factor subunits p65, cRel, and RelB, but not NF-kappaB1 (p105/p50) or NF-kappaB2 (p100/p52). Accordingly, in Salmonella-infected cells, p65 was cleaved dependent on gtgA, gogA, and pipA leading to inhibition of NF-kappaB signalling. To investigate the molecular basis for substrate recognition, mutational analysis of residues in close proximity to the p65 cleavage site (G40/R41) was done and showed that the P1’ residue (R41 in p65) is a critical determinant of substrate specificity. In NF-kappaB1 and NF-kappaB2, a proline residue is present at the corresponding site and this residue prevents cleavage by GtgA, GogA, and PipA. I also present the crystal structure of GtgA alone and in complex with the N-terminal domain of p65. The crystal structure demonstrates the importance of the P1’ residue in substrate specificity and supports a model of DNA mimicry as the mechanism of substrate recognition. This thesis therefore provides detailed insight into the functions and mechanism of substrate recognition, for a family of previously uncharacterised Salmonella virulence proteins.