The course itself, which has changed distance, route, and elevation many times since its inaugural run in 1986, currently consists of a 20-mile (32 km) loop with no aid stations except water at two points along the route and the runner's parked car at the beginning of the loop. Runners of the 100 Mile version run this loop five times, with loops three and four being run in the opposite direction and loop five being runner's choice. Runners of the 60 mile 'fun run' (considered to be harder than Hardrock) complete three circuits of the loop.
With 54,200 feet (16,500 m) of accumulated vertical climb, the 100 mile run is considered to be one of the more challenging ultramarathons held in the United States, if not the world.
In addition to running, competitors must find between nine to eleven books, the number varies per year, and remove the page corresponding to the runner's race number from each book as proof of completion.
The cut-off time for the 100 mile race is 12 hours per loop, and the cut-off for the 60 mile version of the race is 40 hours overall, which averages out to approximately 13 hours and 20 minutes per loop. Since the race's inception in 1986, only fourteen runners out of about 800 have completed the 100 mile race within the official 60 hour cut-off (Mark Williams 1995, David Horton and Blake Wood 2001, Ted "Cave Dog" Keizer 2003, Jim Nelson and Mike Tilden 2004, Brian Robinson 2008 (55:42:27), Andrew Thompson 2009, Jonathan Basham 2010, Brett Maune 2011, Brett Maune 52:03:08 (new course record), Jared Campbell 56:00:15, John Fegyveresi 59:41:21 for 2012, Nick Hollon 57:39:24, Travis Wildeboer 58:41:45 for 2013, Jared Campbell 57:53:20 for 2014). In 2006 nobody finished even the 60 mile 'fun run' in under 40 hours. The best women's achievement is Sue Johnston's 66 miles (106 km) in 2001. More than 30 competitors failed to reach first book (two miles).  
The race is limited to 35 runners and usually fills up quickly the day registration opens. Potential entrants must complete an essay on "Why I Should be Allowed to Run in the Barkley." The race starts at different times each year and is signaled by the lighting of a cigarette.
The course was designed by Gary Cantrell. His idea for the race was inspired upon hearing about Martin Luther King, Jr's assassin James Earl Ray escaping from prison, and making it only 8 miles (13 km) after running 55 hours in the woods. Cantrell said to himself "I could do at least 100 miles." Thus, the Barkley Marathons was born.
- Lisa Jhung, The Impervious Barkley Marathons, Runner's World, April 6, 2010. Retrieved April 04, 2012.
- Matthew Everett, The Barkley Marathons Is the Toughest Race You’ve Never Heard Of, MetroPulse, April 13, 2011
- Matt Mahoney's Barkley Marathons Site
- Washington Post article - "Why We Compete"
- The Believer magazine article - "The Immortal Horizon"
- New York Times article on the race
- International Herald Tribune article on the race
Furtaw, Ed. (2010) Tales From Out There: The Barkley Marathons, The World's Toughest Trail Race, CreateSpace. ISBN 1-4505-4701-X