As critical regulators of vesicular trafficking, Rab proteins comprise the largest GTPase family, with thirty-eight functionally distinct members and another twenty isoforms in the human genome. Activated Rab GTPases interact with effector proteins involved in vesicle formation, transport, tethering, docking and fusion. The specificity of Rab interactions with effectors and regulatory factors plays a central role with respect to the fidelity of membrane trafficking. Rab recognition determinants and the mechanisms underlying interactions with structurally diverse regulatory factors and effectors are complex and poorly understood. Using Rab5 mediated endocytic transport as a model system, the work described in this thesis provides insight into the structural basis underlying the interaction of effectors and regulatory factors with Rab GTPases. In addition, structural and biochemical approaches have been used to define how specific Rab5 interacting proteins function in the endocytic and recycling pathways. These results establish novel structural and functional concepts that can be tested using family wide analyses of Rab GTPase recognition determinants and regulatory roles in the cell.