There are a number of long rivers in the world, but it can be unexpectedly difficult to determine the length of a river. Since many rivers join together in river systems, it isn't always easy to say where one individual river begins. In many cases, the length of a river is measured from the headwaters that are farthest from the river's mouth; in some instances, this means that the "source" of the river is on a tributary that is known by a different name than that of the main river.

Of the longest rivers in the world, one goes through ten countries. The Nile travels through Burundi, Democratic Republic of Congo (DRC), Egypt, Eritrea, Ethiopia, Kenya, Rwanda, Sudan, Tanzania, and Uganda. Other rivers, like the Mississippi, go through only one country.

While the Amazon is usually considered to be the second longest river in the world, it carries more water than any other river. The second and third longest rivers in the Amazon system, the Madeira and the Purus, respectively, are also some of the longest rivers in the world, as are two major portions of the Mississippi-Missouri system.

Australia doesn't have an individual river in the top 20 longest rivers in the world. The longest river in Australia is the Murray River at about 1,475 miles (2,375 km). When combined with other rivers in the Murray-Darling River system, however, the length grows to at least 2,094 miles (3,369 km), putting it on the top 20 list.

Of the top twenty longest rivers in the world, only one is in Europe, while only three are in Africa. The next longest river in Europe, after the Volga, is the Danube, at about 1,771 miles (2,850 km). Nearly half of the twenty longest rivers in the world are in Asia.