There are six basic resources in Rise of Nations. These resources form the foundation upon which a nation builds itself and allows for the creation of units, buildings, and technologies. These resources are able to be gathered or earned by the player by using Citizens to harvest them (Food, Timber, Metal, and Oil), using Caravans and increasing territory (Wealth), or using Scholars (Knowledge). Metal and Knowledge become available once the nation reaches the Classical Age. Oil becomes available once the nation reaches the Industrial Age.