MIDWAY, FL - Hundreds of Thousands of people line up on the streets for the 121st Annual Boston Marathon.
More than 30,000 runners are competing this year.
Boston's Mayor said security will be tight.
This year, they are using drones to help them keep an eye on the race.
It's been four years since the 2013 bombings.
Three people died and more than 200 others were injured when a pair of bombs went off near the finish line.
Today, we look back at pictures from Boston Marathons of the past and how it's world-reknowned.
Onlookers watch participants of the 1930 marathon.
Styliannos Kyriakides of Greece winning the 1st place title in 1946.
There's also a picture of Kyriakides tackling Heartbreak Hill in Newton, this is said to be the most difficult part of the race because it is the last stretch of the run and at 20.5 miles it can be a killer.
Two South Korean men trained for the Boston Marathon in 1947.
South Korea won the marathon that year when Suh Yun-Bok crossed the finish line with a world record-breaking time.
Marathon Legend John Kelley participated in the 1962 marathon.
Although, he didn't win that year, he won in 1935 and 1945.
He also set a record for completing a total of 61 Boston Marathons.