In physiology, the all-or-none law (sometimes the all-or-none principle or all-or-nothing law) is the principle that if a single nerve fibre is stimulated, it will always give a maximal response and produce an electrical impulse of a single amplitude. If the intensity or duration of the stimulus is increased, the height of the impulse will remain the same. The nerve fibre either gives a maximal response or none at all.

It was first established by the American physiologist Henry Pickering Bowditch in 1871 for the contraction of heart muscle.

An induction shock produces a contraction or fails to do so according to its strength; if it does so at all, it produces the greatest contraction that can be produced by any strength of stimulus in the condition of the muscle at the time.

This principle was later found to be present in skeletal muscle by Keith Lucas in 1909. The individual fibres of nerves also respond to stimulation according to the all-or-none principle.