Walking is the main form of animal locomotion on land, distinguished from running and crawling. (When executed in shallow water, it is usually described as wading.) The word is derived from the Old English walcan (to roll).

Walking is generally distinguished from running in that at most 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, often resulting in disqualification even at the Olympic level.) For horses and other quadrupedal species, the running gaits may be numerous, and walking keeps three feet at a time on the ground.

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

• 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 canes/walking sticks, staves, 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); and
• scrambling, which 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 predictably unstable when supported solely by the legs.

For humans, walking is the main form of transportation without an inanimate vehicle or riding animal. A pedestrian is a walking person, in particular on a road (if available on the sidewalk/path/pavement).

Many people walk as a hobby, and in our post-industrial age it is often enjoyed as a form of exercise. Fitness walkers and others may use a pedometer to count their steps. The types of walking include bush walking, racewalking, hillwalking, volksmarching, Nordic walking and hiking on long-distance paths. In some countries walking as a hobby is known as hiking (the typical North American term), rambling (a somewhat dated British expression, but remaining in use because it is enshrined in the title of the important Ramblers' Association), or tramping (the invariable term in New Zealand). Hiking is a subtype of walking, generally used to mean walking in nature areas on specially designated routes or trails, as opposed to in urban environments; however, hiking can also refer to any long-distance walk. More obscure terms for walking include "to go by Marrow-bone stage", "to ride Shank's pony" or "to go by Walker's bus." Walking in a shopping mall is often called "trolling."

The average child achieves independent walking ability between 9 and 15 months old.

The world's largest registration walking event is the International Four Days Marches Nijmegen. The annual Labor Day walk on Mackinac Bridge draws over 60,000 participants. The Chesapeake Bay Bridge walk annually draws over 50,000 participants. Walks are often organized as charity events with walkers seeking sponsors to raise money for a specific cause. Charity walks range in length from 2 mile or 5 k walks to as far as 50 miles (80 km). The MS Challenge 50 is an example of a 50 mile walk which raises money to fight muscular dystrophy.

In Britain, the Ramblers' Association is the biggest organisation that looks after the interests of walkers. A registered charity, it has 139,000 members.