They can be seen up to 20 miles away and will be left on until 13 April.
Valerie Webb switched on the Tribute of Light
Thirteen-year-old Valerie Webb, whose father was killed in the terrorist attacks of 11 September, switched on the tribute in New York. Her father, Nathaniel, was a policeman and his body still hasn't been found.
'No place to hide'
Mourners in New York held two moments of silence, to mark the exact times the two planes crashed into the World Trade Center.
Memorial services were held in other parts of America to remember the 3,000 people who died in the tragedy.
US President George Bush said terrorists would have "no place to hide" as America started its second phase of the war on terror.