Abstract: This dissertation is an examination of Achaemenid monumental gateways built between c. 546 and 331 BCE at the three Achaemenid sites of Pasargadae, Susa and Persepolis. These gates were unique in that they were in essence large columned reception halls that also functioned at portals to these three important Achaemenid sites. Pasargadae, Susa and Persepolis offer the only known surviving examples of Achaemenid gateways and their development is traced over two centuries of time. While Achaemenid architecture and design has been examined in previous studies, this work focuses on the gateways as a means to understanding important elements of Achaemenid imperial construction and design. Examined in chronological order, the development and function of these gates is placed in the wider context of Achaemenid architectural, artistic and ceremonial programs. While serving as monumental entrances, these buildings were intrinsically linked to the buildings to which they led, as well as being integrated to site's plan as a whole. A clear development is traced from the first gate examples at Pasargadae, which show connections to the earlier Neo-Assyrian world, down to the standardization of gateways under the reign of Darius I and Xerxes I at Susa and Persepolis as well as those of later rulers. In conclusion, as with so many Achaemenid imperial buildings, these gates demonstrate a clear and well planned program that projects an image of stability, coherence and cooperation in the Achaemenid world.