Trail running is a sport/activity which consists of running and hiking over trails. In the United Kingdom and Ireland it is also called mountain or fell running although the two types do differ. It differs from road running and track running in that it 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.
The amount of organized trail races has grown 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 a large increase in competitors in non-traditional/off-road triathlons and adventure racing over the past five years.