Trail running is a sport which consists of running and hiking over trails. In the United Kingdom and Irish Republic it is called mountain or fell running. 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 ascents and descents. 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.
The ranks of trail runners are increasing annually. They have grown from 4.5 million to more than 6 million in the United States alone between 2006 and 2012. The amount of organised trail races has grown exponentially over the past few years throughout the world, now well into the hundreds in North America alone. Runners often cite less impact stress compared to road running, as well as the landscape and non-urban environment, as primary reasons for preferring trail running. This move to nature is also reflected in large increase in competitors in non-traditional/off-road triathlons and in adventure racing over the past five years.
A growing number of people are participating in solo backcountry trail running trips, which are an ultralight form of backpacking. An ordinary backpacking expedition may last for eight days, and average eight to ten miles per day with participants carrying fifty to sixty pound backpacks. A 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 less common than ordinary backpacking, is very difficult, and can be dangerous, but it is growing in popularity.
Mountain or Fell running, also known as hill running, is the sport of running and racing, off road, over upland country where the gradient climbed is a significant component of the difficulty. The name arises from the origins of the English sport on the fells of northern Britain, especially those in the Lake District. Fell races are organized on the premise that contenders possess mountain navigation skills and carry adequate survival equipment as prescribed by the organizer.
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.
Because of the natural or serene setting, trail running is viewed as a more spiritual activity than roadside running or jogging. Another reason for growth and popularity is the continual acknowledgment of environmentalism. There is a stress among many trail-race organizers to keep these races "green" or environmentally friendly and minimize disturbance within the environment. 
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, insect repellant 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.
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 makes it difficult to compare performance across different courses. 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, trail running stage races also exist. These multiday stage races usually offer complete support and runner amenities between stages. There are, however, stage races that provide no support apart from water and medical aid, and require competitors to carry all their equipment (food, sleeping bag, change of clothes, emergency flare, compass, flashlight,...). The best-known example of such races is the Marathon des Sables, that was first held in 1986.
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. There are, however, many wildly popular races such as the Ultra-Trail du Mont-Blanc in Europe or the Western States Endurance Run in the United States that have been forced to limit entries due to overwhelming demand.
Some trail races include:
- Fish River Canyon Ultra Marathon - Namibia
- Kalahari Augrabies Extreme Marathon
- Peninsula Ultra Fun Run: 80 kilometres (50 mi)
- Rhodes Trail Run
- Trans Japan Alps Race: 415 km (since 2002)
- Tsuneo Hasegawa Cup Japan Mountain Endurance Race: 71.5 km (since 1993)
- Ultra-Trail Mount Fuji (UTMF): 161 km (since 2012)
- Grand Trail des Templiers: 72 kilometres (45 mi)
- Grande Trail Serra d' Arga: 45 kilometres (28 mi)
- Madeira Island Ultra Trail: 116 kilometres (72 mi)
- Transvulcania: 83 kilometres (52 mi)
- Ultra-Trail du Mont-Blanc: 166 kilometres (103 mi)
- Ultra Trail Serra da Freita: 65 kilometres (40 mi)
- Ultra Trail Geira Romana: 53 kilometres (32 mi)
- Ultra Trail Lago d'Orta: 85 kilometres (53 mi)
- Ultra Trail Amigos da Montanha: 62 kilometres (39 mi)
- Tor des Géants: 330 kilometres (210 mi)
- Al Andalus Ultimate Trail: 230 kilometres (140 mi)
- Badwater Ultramarathon: 135 miles (217 km)
- Bear 100 Mile Endurance Run: 100 miles (160 km)
- Hardrock Hundred Mile Endurance Run: 100 miles (160 km)
- Leadville Trail 100: 100 miles (160 km)
- Mogollon Monster 100: 106 miles (171 km)
- Western States 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)
- OUTDOOR RECREATION PARTICIPATION STUDY 2013 - http://www.outdoorindustry.org/images/researchfiles/ParticipationStudy2013.pdf
- See http://www.ultramarathonrunning.com/races/
- "A Special Report on Trail Running". Outdoor Industry Foundation. 2010. Retrieved 2010-07-09.
- Farago, Susan. "From the Growth of Trail Running to Environmentalism". Austin Fit Magazine. Retrieved 3 December 2013.
- "ATRA history". American Trail Ruunning Assciation. Retrieved 2012-08-25.
- Trail running on the Open Directory Project
- The Beauty of the Irrational - Trail Running through the Fish River Canyon
- ULTRAmarathonRunning.com Global Ultramarathon Races & Events Calendar
- History of Trail and Mountain Running - Trail Run Magazine AU/NZ
- Trail tips and stories