There are two types of ultramarathon events: those that cover a specified distance, and events that take place during specified time (with the winner covering the most distance in that time). The most common distances are 50 and 100 kilometers (or 31.07 and 62.14 miles).

The "100 kilometers" is an official IAAF event.

Other distances/times include double marathons, 24-hour races, and multiday races of 1000 miles or even longer. The format of these events and the courses vary, ranging from single or multiple loops (some as short as a 400-meter track), to point-to-point road or trail races, to cross-country rogaines. Many ultramarathons, especially trail challenges, have severe course obstacles, such as inclement weather, elevation change, or rugged terrain. Many of these races are run on dirt roads or mountain paths, though some are run on paved roads as well. Usually, there are aid stations every 5 to 15 km apart, where runners can replenish food and drink supplies or take a short break.

Timed events range from 6, 12, and 24 hours to 3 and 6 days (known as multi-day events). Timed events are generally run on a track or a short road course, often one mile or less.

The International Association of Ultra Runners (IAU) organises the World Championships for various ultramarathon distances, including 50K, 100K, 24 hours and ultra trail running. These events are sanctioned by the International Association of Athletics Federations (IAAF), the world governing body of track and field. Many countries around the world have their own ultrarunning organizations, often the national athletics federation of that country, or are sanctioned by such national athletics organizations. World records for distances, times and ages are tracked by the IAU.

Ultramarathons are run all over the world and over 70,000 people complete ultramarathons every year.

Several ultra distance events are held in Africa. South Africa hosts the world's oldest and largest ultramarathon, the 90 km Comrades Marathon. Approximately 12,000 runners complete Comrades each year, with over 24,500 in 2000. It also hosts the 56-kilometer Two Oceans Marathon in Cape Town in the southern autumn which attracts approximately 7000 runners. Marathon des Sables is a 7 day stage race which covers 250 km through the Sahara desert in Morocco. The Sahara Race in Egypt, part of the 4 Deserts series, is held annually with about 150 competitors from 30 countries competing. There is also an ultramarathon of 250 km across the Namib desert.

Ultrarunning has become popular in Asia recently, and countries such as Taiwan, Japan, and Korea have hosted IAU World Championships in the last few years. Korea's first ultramarathon was held in 2000. India's first ultra marathon was held in 2007, in Bangalore. The Gobi March , first held in 2003, in northwest China was China's first ultramarathon. Singapore has a double marathons night race called Sundown Marathon since 2008. In Nepal, the Kanchenjunga Ultra Marathon Trail Running Race , first held in 2009, started from Phunlin Bazaar (near Taplejung) at the Nepal and Sikkim border. Mongolia hosts during June of each year the Mongolia Sunrise to Sunset marathon (www.ultramongolia.com/).

The first ultramarathon held in New Zealand was on a 100 km track. Australia and New Zealand are host to some 100 ultramarathons each year. An Australian ultramarathon was the Westfield Ultra Marathon, an annual race between Sydney and Melbourne which was contested between 1983 and 1991. Greek runner Yiannis Kouros won the event five times during that period. Australia is also the home of one of the oldest six-day races in the world, the Cliff Young Australian 6-day race, held in Colac, Victoria. The race is held on a 400-meter circuit at the Memorial Square in the centre of Colac, and has seen many epic battles since its inception in 1984. The 20th Cliff Young Australian 6-day race was held between 20–26 November 2005. During that event Kouros beat his existing world record 6-day track mark and set a new mark of 1036.851 km. The Coast to Kosciuszko inaugrated in 2004, is a 246 kilometres (153 mi) marathon from the coast to the top of Mount Kosciuszko, Australia's highest mountain.

Ultrarunning is popular in Europe, where over 200 ultramarathons are held each year. Among the biggest are: