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Guanylate binding proteins (GBPs) are large GTPases that are substantially upregulated by interferons during infection. The human genome consists of seven GBP family members with high sequence identity. GBPs have been implicated to confer host resistance to a number of pathogens across several species. In mice, specific GBP family members are responsible for host defence mechanisms, including the induction of inflammasome responses during bacterial infections, and the disruption of pathogen vacuoles leading to effective protection against the parasite Toxoplasma gondii, and the bacteria Salmonella enterica typhimurium and Chlamydia trachomatis. Toxoplasma is an apicomplexan intracellular parasite that resides within a parasitophorous vacuole (PV), and can cause severe complications and even death in humans and other animals. The aim of this project was to analyse the characteristics and roles of individual human GBP family members in cells at steady state and in Toxoplasma infected cells. The first step was to develop tools to study the proteins, including producing and characterising specific antibodies, establishing cell overexpression systems and characterising cells deficient in certain GBPs. Using these tools, the subcellular localisations of GBP1 and GBP4 were determined to the cytoplasm and nucleus respectively. It was concluded that during type I and II Toxoplasma infection GBP1 and 4 are not recruited to the PV like in the mouse. Despite this, in human epithelial cells, GBP1 plays an important and specific role in the restriction of Toxoplasma replication. It was deduced that GBP4 protein levels are dramatically reduced during infection with the type I, but not the type II strain of Toxoplasma. GBP4 protein levels could be stabilised during type I Toxoplasma infection with an inhibitor of cysteine, serine and threonine proteases. Using an antibody specific for GBP1 and 2, a large dataset of potential interaction partners in a Toxoplasma strain-specific fashion was generated. The tools produced, specifically the GBP-specific antibodies, provide a valuable resource that can be used by other lab members and collaborators to more fully understand the functions of these interesting and important large GTPases.