At its peak, the Seleukid empire stretched from the Aegean to the borders of India. In this vast area existed regions with quite different physical characteristics and climates, which naturally had a bearing on the sizes of populations that could be supported and their economic activities.

For the purposes of this study, five major regions are considered (see map), which not only had a certain uniformity within themselves, but also some continuity in their association with the Seleukids. The original core of the empire was Mesopotamia, to which were soon added the Upper Satrapies – the eastern regions as far as India – followed by northern Syria and Kilikia. Asia Minor did not have a continuous history of Seleukid rule and the last major region to be acquired was Koile Syria, that is, southern Syria, Phoenicia and Palestine.

Because of the scarcity of evidence, the analysis that follows is rather sketchy, but aims nevertheless at rough regional estimates of population, which were obviously one of the determining factors in the Seleukid economy.

Since population figures are clearly going to be the subject of much disagreement, it should be emphasized that great accuracy is not a requirement here. What is presented, based on the available evidence, is simply a best estimate of a range of figures. While there is a certain probability that any other figure outside the range may be true, this probability becomes increasingly smaller the further one diverges from the best estimate.