More than 36,000 runners are due to start the 2014 Boston Marathon.
One year ago two bombs exploded near the finish line killing three people and wounding more than 260.
A police officer was also killed during the hunt for the perpetrators.
The event has attracted 9,000 more runners this time round and security is tight:
It will be an emotional day for athletes and spectators:“For us it’s all about renewal … reclaiming our rightful place at the finish. The excitement is just so heart palpitating – I just – there is no way to encapsulate it.” said one Bostonian runner.
For a Canadian athlete it is about defiance: “The bad guys aren’t gonna take this race away from us and all the people that get involved in it. It’s good to be back doing it again.”
Two brothers Tamerlan and Dzhokhar Tsarnaev are suspected of the of attack. Tamerlan was killed in a police shootout his 20-year-old brother is due to stand trial in November.