Walking (also called ambulation) is one of the main gaits of locomotion among legged animals, and is typically slower than running and other gaits. Walking is defined by an 'inverted pendulum' gait in which the body vaults over the stiff limb or limbs with each step. This applies regardless of the number of limbs - even arthropods with 6, eight or more limbs.

In humans and other bipeds, walking is generally distinguished from running in that only one foot at a time leaves contact with the ground and there is a period of double-support. In contrast, 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 quadrupedal species, there are numerous gaits which may be termed walking or running, and distinctions based upon the presence or absence of a suspended phase or the number of feet in contact any any time do not yield mechanically correct classification. The most effective method to distinguish walking from running is based on the percent of the stride in which a foot is in contact with the ground (averaged across all feet); defining a walk as greater than 50% contact corresponds well with identification of 'inverted pendulum' mechanics via force plate measurements.

The average human child achieves independent walking ability around 11 months old. The word walk is descended from the Old English wealcan "to roll".
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