Since then, it's become the most celebrated road race on Earth.   The 26.2 mile trek takes runners through eight cities and towns - Hopkinton, Ashland, Framingham, Natick, Wellesley, Newton, Brookline, and Boston.

This year's edition has drawn 30,000 competitors from 99 countries -- the third-largest field in the history of the race. Principal sponsor John Hancock Financial will award $830,500 in prize money.

Cheering them on will be an estimated 1 million spectators.

>> CLICK HERE TO SEE THE 2016 RACE COURSE MAP

The oldest participant this year is 84-year-old Jules Wikler from Medford, N.Y. Paul Goldfinger from Newton is the youngest at 18 years, 2 months and 18 days.

Authorities say security will be tight again this year after the recent attacks in Brussels, though officials know of no specific threat against the marathon.