The Boston Marathon is the oldest annual marathon in the world and dates back to 1897, when it first ran after inspiration from the 1896 Olympics event.
It is considered America's most prestigious marathon, and draws competitors and spectators from all over the world.
It takes place to coincide with Patriots' Day every year, the third Monday in April, which commemorates the anniversary of the first battles of the American Revolutionary War.
The spectacle regularly attracts more than 20,000 participants and 500,000 spectators each year and the final mile of this year's event was dedicated to victims of Newtown shootings.
Originally a local event, it swiftly attracted runners from all over the world and was free for most of its history, with the only prize being a wreath woven from olive branches. However, corporate-sponsored cash prizes was awarded in the 1980s, when professional athletes began to refuse to run the race without cash awards and the first financial prize for winning the marathon was awarded in 1986.
In 2011 Geoffrey Mutai of Kenya ran the fastest marathon ever in a time of two hours, three minutes and two seconds at the Boston event but, because the route allows a tailwind, was not recognised as an official world record.
Margaret Okayo, also from Kenya, set the women's Boston Marathon course record with a two hour, twenty minute and 43 second performance in 2002.
Last year, Rick Hoyt, who has cerebral palsy, finished his 30th Boston Marathon alongside his father Dick, who was 72 years old.
The race also has a strong social history. During the Korean War, Korean-Americans were banned from the race and women were not officially allowed to enter until 1972 although in 1967, Kathrine Switzer registered as "KV Switzer" and successfully ran the race.