is therefore interested in this period as a crucial step in the evolution of society—a long and cumulative process whose roots could be seen at the beginning of the Neolithic more than 6000 years earlier and which had picked up steam in the preceding Ubayd period in Mesopotamia. This is especially the case in English-language scholarship, in which the theoretical approaches have been largely inspired by anthropology since the 1970s, and which has studied the Uruk period from the angle of 'complexity' in analysing the appearance of early states, an expanding social hierarchy, intensification of long-distance trade, etc. In order to discern the key developments which make this period a crucial step in the history of the ancient Near East, research focusses mainly on the centre, Lower Mesopotamia, and on sites in neighbouring regions which are clearly integrated into the civilization which originated there (especially the 'colonies' of the middle Euphrates). The aspects traced here are mostly those of the Late Uruk period, which is the best known and undoubtedly the period in which the most rapid change took place—it is the moment when the characteristic traits of the ancient Mesopotamian civilization were established. Technology and economy The 4th millennium BC saw the appearance of new tools which had a substantial impact on the societies that used them, especially in the economic sphere. Some of them, although known in the preceding period, only came into use on a large scale at this time. The use of these inventions produced economic and social changes in combination with the emergence of political structures and administrative states. Agriculture and pastoralism In the agricultural sphere, several important innovations were made between the end of the Ubayd period and the Uruk period, which have been referred to in total as the 'Second Agricultural Revolution' (the first being the Neolithic Revolution). A first group of developments took place in the field of cereal cultivation, followed by the invention of the ard—a wooden plough pulled by an animal (ass or ox)—towards the end of the 4th millennium BC, which enabled the production of long furrows in the earth. This made the agricultural work in the sowing season much simpler than previously, when this work had to be done by hand with tools like the hoe. The harvest was made easier after the Ubayd period by the widespread adoption of terracotta sickles. Irrigation techniques also seem to have