This chapter presents demographic conditions as a determinant for economic performance in the ancient Greco-Roman era. It focuses on the relationship between demographic structures and macro-economic features. The chapter first outlines the fundamental demographic characteristics of the Greco-Roman world. Next, it presents a theoretical model of the interdependence of economic and demographic development. Finally, the chapter explores the model's principal variables in the context of ancient Mediterranean economies. Mean life expectancy at birth is conventionally put in the range of 20-30 years. From the middle of the second millennium BC to the early first millennium AD, all parts of the Mediterranean and its hinterlands experienced significant demographic growth. The baseline upward trend in ancient population number is modulated by two distinct layers of variation in the short and medium terms. Several features including mortality, morbidity, nutrition, housing and clothing, point to significant improvements in well-being in the Greek Aegean between the tenth and the fourth centuries BC.