As an English name, Chloe has been in use since the Protestant Reformation. It started getting more popular in the 1980s in the United Kingdom and then the United States. It was the most popular name for girls in England and Wales from 1997 to 2002. This is one of the few English-language names that is often written with a diaeresis, as Chloë.
As an English name, Monica has been in general use since the 18th century. In America it reached the height of its popularity in the 1970s, declining since then. A famous bearer was the Yugoslavian tennis player Monica Seles (1973-).
As an English name, it became popular due to the Fleetwood Mac song Rhiannon (1976), especially in the United Kingdom and Australia.