Legionella pneumophila is the causative agent of a severe form of pneumonia known as Legionnaires’ disease. Legionella utilizes a type IV secretion system, encoded by the dot/icm gene cluster, to secrete circa 300 effectors that highjack host cellular function to allow the bacterium to live intracellularly inside macrophages. Although the molecular basis of effector function has been elucidated for a small number of effectors, there remain a large number of effectors for which no function has been ascribed. Moreover, when function is known, the host target to which the effector binds has not always been identified. Finally, when effector function and target are known, the molecular basis of effector-target interaction often remains to be determined. This proposal aims to address these issues using a structural biology approach. Another issue addressed by this proposal is the structural and molecular characterisation of the Dot/Icm type IV secretion system responsible for effector secretion through the double membrane of Legionella. Therefore, we propose here three complementary lines of research: i- using a structural biology approach, we will determine the structures of functionally-uncharacterised Legionella effectors in order to derive their function; any function derived from structure will be validated by Professor Craig Roy, a leading researcher in the field of Legionella infection. ii- when/once the effector’s target is known, we will investigate the molecular basis of effector-target interaction by determining the structure of the binary effector-target complexes. iii- we will investigate the structure of the secretion apparatus that is responsible for secreting effectors through the Legionnella membrane. This proposal will greatly enhance our knowledge of the various mechanisms used by pathogenic bacteria to develop intra-cellularly in humans, and provide the basis for drug design efforts aiming at neutralizing Legionella infection.