Feeding the Capital
Contributed by David Verne
At this time in the Empire's history, Rome had a population of 1.0 - 1.6 million; this will be the largest any European city reached until London during the Industrial Revolution. Their diet primarily consisted of bread, olive oil, and wine, with an estimated 330,000 tons of grain needed annually. Italy would never have been able grow the amount of grain needed, so trade was used to fill the demand. North Africa and Egypt had always been large producers of grain, and with Roman rule over the entire Mediterranean Sea, the amount produced and exported each year grew. In the waning years of the Republic, the Romans had wiped out all pirates in the sea, making natural dangers the only problem for ships. During the Principate, the current system of government where an Emperor ruled but acted as merely the first among equals, the Empire stayed out of the way of the market, and only stepped in if famine or similar problems disrupted trade. This allowed the merchant class to flourish, and many companies were established with shareholders and functioned similarly to corporations today. This ease of trade enabled many cities to exist even though they couldn't produce enough food for themselves, similar to modern cities. 
- Temin, Peter (2013). The Roman Market Economy. Princeton University Press. ISBN 978-0- 691- 14768- 0.