Regulation is the process by which a watchmaker adjusts the rate of a mechanical movement — how fast or slow it runs — to bring it as close as possible to keeping perfect time. In a conventionally regulated movement, this is achieved by moving the regulator index: a lever with two pins that straddle the outer coil of the hairspring, effectively changing the spring's active length. Moving the index to lengthen the active spring slows the oscillation; shortening it speeds the oscillation up. The index is typically engraved with + and − markers to guide the adjustment.

In a free-sprung movement, regulation is performed differently — by adjusting small eccentric screws or weights on the balance wheel rim, which alters the wheel's moment of inertia and therefore its oscillation frequency. This method is more time-consuming but produces a more stable result, as the hairspring is left completely free.

Regulation is performed on a timing machine — an instrument that measures the watch's rate in beats per second and displays it as seconds gained or lost per day. A thorough regulation involves measuring the rate in multiple positions (dial up, dial down, crown up, crown left, crown right, crown down) and averaging across them, since gravity acts differently on the balance depending on orientation. COSC certification, which must be passed for a movement to be called a chronometer, requires the movement to run within −4/+6 seconds per day across all positions and temperatures.