Even though many changes have been applied to estimates of income in ancient and medieval times, the growth patterns between c. 500 bce and 1000 ce have essentially remained unaltered. This changed around the tenth and fourteenth century ce, when the urbanized parts of Italy were the first to overtake China’s per capita gross domestic product (GDP), a development referred to as the “Great Crossing”. In this chapter, we disentangle the various reasons, divided into seven groups. First, we look at the Palaeolithic period until 500 bce to distinguish long-run patterns. Second, given the predominantly Malthusian nature of general economies in this phase, drastic reductions or increases of population could result in increases or decreases of GDP per capita. Third, social relations and class structure varied strongly among regions, thus influencing commercialization and technological change. Fourth, the share of manufacturing in the economy, which has, in general, a higher per capita productivity than agriculture, might also cause differences in income. Fifth, the structure of agriculture could also affect income. For example, a higher share in pasture might reduce income expressed in calories but increase it in monetary values. Calculations for Italy, Byzantium, and Mesopotamia find that the difference in per capita income is about 45%, explained by agriculture and manufacturing. Sixth, climate might affect productivity. Seventh, technological development played an important role in economic growth, where Europe took over from East Asia in the early modern period.