a group of sites distributed over an immense area, covering all of Mesopotamia and the neighbouring regions up to central Iran and southeastern Anatolia. The Uruk culture itself is certainly characterised mainly by sites of southern Mesopotamia and others which seem to have directly resulted from migrations from this region (the 'colonies' or 'emporia'), which are clearly part of the Uruk culture. But the phenomenon which is known as the Uruk expansion is detected on sites situated across a vast zone of influence, covering the whole Near East, regions which were not all really part of the Uruk culture, which was strictly-speaking limited to Lower Mesopotamia. The relations of some areas with the Uruk culture are very unclear, such as the little-known cultures of the Persian Gulf in this period, and Egypt whose exact relations with the Uruk culture were distant and are the object of debate, as well as the Levant, where the influence of southern Mesopotamia remains barely perceptible. But in other areas the Uruk culture is more evident, such as Upper Mesopotamia, northern Syria, western Iran and southeastern Anatolia. They generally experienced an evolution similar to that of lower Mesopotamia, with the development of urban agglomerations and larger political entities and they were strongly influenced by the culture of the 'centre' in the later part of the period (c. 3400–3200), before a general strengthening of their own regional cultures took place at the turn of the 3rd millennium BC. The interpretation of the expansion of the Uruk culture into neighbouring regions poses numerous problems and many explanatory models (general and regional) have been proposed in order to explain it. Susiana and the Iranian Plateau The region around Susa in the southwest of modern Iran, is located right next to lower Mesopotamia, which exercised a powerful influence on it from the 5th millennium BC, and might be considered to have been part of the Uruk culture in the second half of the 4th millennium BC, either as a result of conquest or a more gradual acculturation, but it did retain its own unique characteristics. The Uruk period levels at Susa are called Susa I (c. 4000–3700 BC) and Susa II (c. 3700–3100 BC), during which the site became an urban settlement. Susa I saw the beginning of monumental architecture on the site, with the construction of a 'High Terrace', which was increased during Susa II to measure roughly