Eukaryotic cells use vesicles to transport material between their cellular compartments. The Golgi complex is the central sorting organelle for most of vesicular trafficking events in the cell. To control these trafficking events, the cell utilizes small GTPases that act as on/off switches. At the Golgi, the GTPase Arf1 coordinates most of the outgoing vesicle formation. Arf1 is regulated by the standard GTPase cycle and is activated when a GEF exchanges GDP for GTP. Once GTP bound, Arf1 undergoes a conformational change that allows it to localize to the Golgi complex and recruit effector proteins.At the Golgi, Arf1 is activated by three different GEFs, Gea1, Gea2, and Sec7. These proteins share a common catalytic GEF domain and regulatory domains of unclear function. While they activate the same substrate, Gea1/2 are functionally distinct from Sec7, which activates Arf1-dependent vesicle formation at later parts of the Golgi. Gea1 and Gea2 activate Arf1 at the early and medial Golgi respectively for COPI vesicle formation. While Gea1 and Gea2 are genetically redundant, they have distinct localization in vivo. To understand the regulation of the Golgi Arf-GEFs, I used a structural approach to determine the function and mechanism of these regulatory domains. I present the first full-length structure of a large Arf-GEF, Gea2, and the structure of Gea2 bound to its substrate Arf1. Using these structures, I was able to structurally and biochemically characterize the activation of Arf1 by Gea2.