A Rudyard Kipling short story "On the City Wall" gets the credit for calling prostitution the world's oldest profession. If it's not really the oldest occupation, it's pretty close to it. Prostitution involves having sexual intercourse with someone in exchange for some kind of payment (money, food, clothing), and it's been around for millennia.
In ancient times, prostitution was often performed within a sacred context, as part of a religious ceremony or for a sacred purpose. In Sumerian and Babylonian cultures (2400 B.C.E. and beyond) having sex with a temple prostitute was one way to gain favor from a fertility god.
In ancient Rome, prostitution was not part of religion. Slaves were often forced to work as prostitutes, as were orphaned children. In ancient Greece, male prostitutes were common, and were mainly adolescent boys. Slave boys typically worked in Athens' male brothels [sources: New World Encyclopedia].
In 2012, there were roughly 42 million prostitutes in the world, three-quarters of them between the ages of 13 and 25, and 80 percent female. Now, as in ancient times, most prostitutes work in this field against their will, and human trafficking is a huge human rights issue [sources: ProCon, Lubin].