Walking (also called ambulation) is the main form of animal locomotion on land, distinguished from running and crawling. When carried out in shallow waters, it is usually described as wading and when performed over a steeply rising object or an obstacle it becomes scrambling or climbing. The word walk is descended from the Old English wealcan "to roll".

Walking is generally distinguished from running in that only one foot at a time leaves contact with the ground; for humans and other bipeds, running begins when both feet are off the ground with each step. (This distinction has the status of a formal requirement in competitive walking events, resulting in disqualification at the Olympic level.) For horses and other quadrupedal species, the running gaits may be numerous, and while walking keep three feet at a time on the ground.

The average human child achieves independent walking ability around 11 months old.

While not strictly bipedal, several primarily bipedal human gaits (where the long bones of the arms support at most a small fraction of the body's weight) are generally regarded as variants of walking. These include:

    * Hand walking; an unusual form of locomotion, in which the walker moves primarily using their hands.
    * Walking on crutches (usually executed by alternating between standing on both legs, and rocking forward "on the crutches" (i.e., supported under the armpits by them);
    * Walking with one or two walking stick(s) or trekking poles (reducing the load on one or both legs, or supplementing the body's normal balancing mechanisms by also pushing against the ground through at least one arm that holds a long object);
    * Walking while holding on to a walker, a framework to aid with balance; and
    * Scrambling, using the arms (and hands or some other extension to the arms) not just as a backup to normal balance, but, as when walking on talus, to achieve states of balance that would be impossible or unstable when supported solely by the legs;

For humans, walking is the main form of transportation without a vehicle or riding animal. An average walking speed is about 6 to 7 km/h (3 to 4 mph), although this depends heavily on factors such as height, weight, age, terrain, surface, load, culture, and fitness. A pedestrian is a person who is walking on a road, sidewalk or path.