Trail running is a sport which consists of running and hiking over trails. It differs from road running and track running in that generally takes place on hiking trails, often in mountainous terrain, where there can be much larger ascends and descends. It is difficult to definitively distinguish trail running from cross country running. In general, however, cross country is an IAAF governed discipline that is typically raced over shorter distances (rarely over 12 kilometers), whereas trail running is loosely governed, and run over longer routes.
Related activities 
There is a growing number of people participating in solo backcountry trail running trips, which are a sort of ultralight backpacking. While an ordinary backpacking expedition may last for eight days, averaging eight to ten miles per day, with participants carrying fifty to sixty pound backpacks, backcountry trail runner will do the same trip in three to four days, covering much greater distances each day, and carrying only minimal equipment. This type of backpacking is rare, as it is very difficult and dangerous, but it is growing in popularity.
According to a 2010 special report on trail running published by the Outdoor Industry Foundation, "4.8 million Americans ages 6 and older participated in trail running in 2009." This research shows a particularly heavy following in the Mountain States, the Western US, and California.
Many trail runners use specially designed shoes that have aggressively knobby soles that are generally more rigid than road running shoes. The usually EVA compound midsole often contain a lightweight, flexible nylon plastic layer to protect the feet from puncture wounds from sharp rocks or other objects. Since trail running take place on softer surfaces (e.g., grass, dirt) than road races, cushioning is not as important so often the shoes are less 'cushioned' than their counterparts designed for tarmac. Additionally, trail running shoes are low to the ground which provides the best stability on uneven terrain.
Other equipment includes wicking garments, water bottles, sunscreen, sunglasses, gaiters, anti-insect spray, and ivy block. Some trail runners attach lightweight crampons to the bottom of their shoes to aid with traction in the snow and on ice. An alternative way to carry water is use a hydration bladder with drinking tube carried in a backpack or waistpack. Carrying the Ten Essentials may reduce the hazards inherent in wilderness travel. Some trail runners use ultra light hiking poles (which are often not allowed during competition) to increase speed and stability.
Trail running races are organised globally. Due to the relatively short history of trail running as an organised sport, there are very few established organising bodies. For example, in the United States, the American Trail Running Association was only founded in 1996 to represent trail races in the US.
Compared to road races, there are often fewer participants as number of entries is often limited. There can be a few reasons for this: narrowness of trails, national parks (where the courses are often set) limits, safety and environmental concerns. Distances in races vary widely, from 5 km, to over 100 miles (161 km). Many high profile trail races are of ultramarathon distance. Races of similar distance often differ significantly in terms of terrain. This make it difficult to compare performance across different course. This is in contrast to times over standard distances in road running, such as 10 km or marathon.
Aid stations supplying food and beverages are commonly located every 5 to 10 kilometers along the course. Most trail races only have a single stage, where competitors are timed over the entire duration of their run, including stops at aid stations. However, fully supported trail running stage races also exist. These multiday stage races offer complete support and runner amenities between stages.
Some trail races include:
- Peninsula Ultra Fun Run: 80 kilometres (50 mi)
- Rhodes Trail Run
- Kalahari Augrabies Extreme Marathon
- Fish River Canyon Ultra Marathon - Namibia
- Ultra-Trail du Mont-Blanc: 166 kilometres (103 mi)
- Madeira Island Ultra Trail: 116 kilometres (72 mi)
- Transvulcania: 83 kilometres (52 mi)
- Ultra-Trail Mount Fuji (UTMF): 161 km (since 2012)
- Tsuneo Hasegawa Cup Japan Mountain Endurance Race: 71.5 km (since 1993)
- Trans Japan Alps Race: 415 km (since 2002)
North America 
- Western States Endurance Run: 100 miles (160 km)
- Leadville Trail 100: 100 miles (160 km)
- Badwater Ultramarathon: 135 miles (217 km)
- Hardrock Hundred Mile Endurance Run: 100 miles (160 km)
- Kepler Challenge: 60 kilometres (37 mi) also includes the Luxmore Grunt 28 kilometres (17 mi) (New Zealand)
- Kokoda Challenge Race: 96 kilometres (60 mi)
See also 
- Trail running at the Open Directory Project
- The Beauty of the Irrational - Trail Running through the Fish River Canyon