Duathlon is an athletic event that consists of a running leg, followed by a cycling leg and then another running leg in a format bearing some resemblance to triathlons. The International Triathlon Union governs the sport internationally.
Relationships to similar sports 
Duathlons are most similar to triathlons, with the key difference being the lack of a swimming leg. Other sports derived from triathlon include aquathlon, which combines swimming and running but has omitted the cycling part, and aquabike, with the swim and bike but no run.
Team Duathlon 
In this variation, the cycling and running segments need to be undertaken simultaneously by a team consisting of a predetermined number of individuals. Only one of the team members is running while all others are cycling. All the members of a team must be together at any given point of time but exchanging the tasks of running and cycling amongst the team members themselves is permitted.
The challenge is to rotate the task of running at an optimum rate such that the time from start to destination is minimized. This demands a smooth coordination amongst all team members which makes the sport more exciting. An example is the 100 km-Duathlon around Dresden which is the oldest of its kind in Germany.
Notable duathlons 
The world's largest duathlon to date is the inaugural London Duathlon, which took place on 17 September 2005 in Richmond Park, Surrey. Approximately 2,500 people took part. The main race consisted of a 10 km run, 20 km cycle ride and 5 km run. Since 2005 The London Duathlon has grown to just under 4,000 participants, 2010 will see the first running of the Ultra Distance at London which will consist of 20 km run, 80 km bike and 10 km run.
The World's premier duathlon is Powerman Zofingen, in Switzerland. It's a hilly 10 km run, mostly on trails, followed by a very hilly 150 km road bike consisting of three 50 km loops each of which feature the Bodenburg ascent (scene of Kenny Souza's DNF in a snow storm when he raced in a neon Speedo and small tank top), and finally a very hilly 30 km two loop trail run. Mark Allen, six time Ironman Hawaii champion, has been quoted as saying Powerman Zofingen is the hardest race he ever participated in.
The longest continually running duathlon series is still organized by the NY Triathlon Club in and around NY City, USA.
With the popularity of extreme sports, a new form of duathlon has become popular in the last few years. Referred to as an off-road duathlon, or "Dirty-Du", it consists of an off-road or trail run, followed by a mountain bike leg followed by a final off-road or trail run. The Dirty Du off-road duathlon series, held in Texas, was made famous by Lance Armstrong, who won it in 2003.
Notable Duathletes 
- Kenny Souza – 1990 ITU world champion
- Benny Vansteelant – winner of the World Duathlon, World Long-Distance Duathlon, European Duathlon and numerous Powerman titles.
- Joerie Vansteelant – winner of the World Long-Distance Duathlon 2007
- Koen Maris – 2007 Powerman Duathlon World Champion (10 km/150 km/30 km)
- Rob Woestenborghs – 2008 World Champion Duathlon Short Distance in Rimini
- Bart Aernouts – 2010 World Champion Duathlon Short Distance in Edinburgh
- Emilio Martin – 2012 World Champion Duathlon Short Distance in Nancy
- Catriona Morrison – 2006 and 2010 World Duathlon Champion, 2007 and 2008 World Long-Distance Duathlon Champion, 2007 and 2009 European Duathlon Champion
- Andrea "RAT" Ratkovic - 2006 World Long-Distance Duathlon bronze medalist, multiple USA Pro National Duathlon Champion, Elite Duathlete of the Year, Rookie Duathlete of the Year, multiple Powerman wins