The ancient economy's most important sector by far was agriculture, which employed the overwhelming majority of the population and produced the majority of economic output in every ancient civilization. All other economic activity — trade, manufacturing, finance — was built on the agricultural base and ultimately dependent on agricultural surplus.

The productivity of ancient agriculture was considerably lower than modern agriculture but not negligible. Greek and Roman agronomists wrote practical treatises on crop rotation, fertilization, irrigation, and crop selection that show sophisticated understanding of agricultural practice. The yields achievable with ancient techniques varied enormously by region, soil quality, and rainfall, but the best ancient agriculture could achieve yields comparable to medieval European farming.

The political economy of ancient agriculture was shaped by the structure of land ownership. The concentration of land in the hands of large landowners (using slave labor) versus the distribution across smallholder peasant families is one of the central tensions in Roman political history. The Gracchan land reform debates were about redistribution of public land occupied by wealthy families; the ultimate failure of reform contributed to the conditions that produced civil war.

The Roman agricultural writers — Cato, Varro, Columella, Palladius — provide the most detailed ancient documentation of specific agricultural practices. Columella's De Re Rustica (1st century AD) is the most comprehensive, covering viticulture, arboriculture, grain farming, animal husbandry, and kitchen gardening in twelve books.