This article discusses the architecture of Assyria and Babylonia, two kingdoms that were located in modern-day Iraq and surrounding parts of Syria, Turkey, and Iran. This region overlaps with Mesopotamia (an ancient Greek name for the region between the Tigris and Euphrates Rivers). The rise to prominence around c. 1800 bce of the cities of Assur in northern Iraq and of Babylon in central Iraq is taken as the article’s starting point. The main focus is, however, on the later histories of Assyria (c. 900–612 bce) and Babylonia (c. 626–538 bce). Both kingdoms can be said to have reached an imperial scale during these periods (Assyria around 730 bce during the reign of King Tiglath-Pileser III, and Babylonia when its armies conquered Assyria in 612 bce). Both empires came to control large parts of western Asia and at times also Egypt. This chapter will, however, focus on Mesopotamia proper, what might be described as its architectural koine (a multiregional shared material culture). The conquest of Babylonia by the Achaemenid Persian armies in 538 bce is taken as the end date. Architecture is an integral part of society and cannot therefore be studied on its own. The discourse on Mesopotamian architecture is notably sparse and uneven (as becomes apparent in this article). The limited nature of the discourse can be explained in several ways. First, although Mesopotamian architects created some of the most renowned buildings of their times, those architects did not write down their ideas, nor did they claim authorship. Ancient textual sources, although abundantly preserved, provide limited information when it comes to architecture. The activity of architecture was instead based on learned practice. Second, the architecture of the region was predominantly constructed of mud bricks supplemented with wood. More-extensive use of stones was generally limited to monumental buildings. Over the centuries, these buildings have collapsed and come to be buried under their own, and later, debris. Generally, only the lowest parts of the ground floor walls have survived. Our knowledge of ancient architecture is therefore dependent on archaeological excavations that commenced in the middle of the 19th century. Third, from the time the first excavations in the region commenced, archaeologists have focused mostly on the big urban centers and their monumental palaces and temples. Archaeologists have become more interested in other types of buildings and settlements over time, but our knowledge remains limited and biased to certain regions and periods. These biases, unfortunately, continue to shape the discourse and limit what can be referenced. Although this chapter does not aim to be comprehensive, it does include a substantial selection of the works that have been published on the architecture of the region.