The moon-phase indication is a device serving to visualize on the watch dial the various appearances adopted by the Earth's satellite in the course of a lunar cycle: it may be waxing, full, waning or entirely hidden by the shadow of the Earth - and thus referred to as a new moon.

These different phases are traditionally represented by a mobile disk bearing two diametrically opposite depictions of the Moon alternately appearing through an aperture. The latter is cut out so as to provide a visual simulation of the Moon's waning and waxing episodes. As soon as the first interpretation vanishes, the second instantly comes on the scene. Moon phases may also be indicated by means of a hand pointing to various symbols, or by a three-dimensional sphere spinning on its axis, and of which one half is light-colored while the other is dark.

The mechanical difficulty of moon phases lies in the unusual interval between two full moons. By the 20th century, this complication had become essentially a romantic feature.