The piano action mechanism (also known as the key action mechanism or simply the action) of a piano or other musical keyboard is the mechanical assembly which translates the depression of the keys into rapid motion of a hammer, which creates sound by striking the strings. Action can refer to that of a piano or other musical keyboards, including the electronic or digital stage piano and synthesizer, on which some models have "weighted keys", which simulate the touch and feel of an acoustic piano. The design of the key action mechanism determines the "weight" of the keys, i.e., the force required to sound a note or striking power. "A professional pianist is likely to care most about the piano's action, because that is what controls its responsiveness and relative lightness--or heaviness--of touch. Roughly speaking, a piano's action is light when its keys fall easily under the fingers, and heavy when a noticeable downward thrust is required. The action, in short, is what makes a piano playable or not to an individual musician."

The string hammer action was the important innovation that Bartolomeo Cristofori created when he invented the pianoforte; Cristofori is credited with building his first instrument by 1700. Although similar hammer actions were devised at about the same time by Marius (1716) and Christoph Gottlieb Schröter (1717), Cristofori was the only one of these three to actually construct an instrument. Alfred Dolge notes the first pianos were quite similar in construction to contemporary harpsichords, while incorporating key features of the clavichord, including the soundboard, metal strings, and percussive-string method of creating sounds.

With the piano, a hammer strikes the string, whereas with a harpsichord, a mechanism plucks the string. Cristofori's improved escapement action (1720) embodied many of the principles still found in 2000-era actions. It used levers to magnify the small motion of the piano key into a large motion of the hammer, and was arranged so that the very last part of the hammer's motion before striking the string would be purely the result of inertia and not propelled by the key; this prevents the key from pressing the felt-covered hammer firmly into the string, which would damp and stop the string vibrations and the sound.

In Cristofori's improved escapement action (1720), when the key is depressed, the other end of the key-lever raises a horizontal hopper (which Cristofori called the linguetta mobile) with a stepped projection; the hopper raises the curved hammer butt. Because of the stepped projection, the hopper slips off the key just before the hammer strikes the string, providing the escapement action. Other modern features are apparent in his diagram, including the presence of a back check to facilitate repetition, and using the rising tip of the key-lever to lift the damper.

Cristofori died in 1731 and left no significant pupils; the instruments that were in process were finished by Giovanni Ferrini, who also is credited with building a pianoforte dated to 1730. However, Gottfried Silbermann built pianos with both Cristofori and Schröter actions. Silbermann produced Schröter action pianos as early as 1728; Silbermann also built two Cristofori action pianos which were submitted to Johann Sebastian Bach; according to Bach's pupil Johann Friedrich Agricola, Bach rejected them for having a weak treble and heavy action, and Silbermann did not build more Cristofori action pianos for two decades, when he built several for Frederick the Great in 1747.

Two of Silbermann's pupils, Johannes Zumpe and Americus Backers, popularized the Cristofori action as the "English" action after moving to London. Zumpe introduced the square piano between 1760 and 1765; Alfred James Hipkins called Zumpe's action "rudimentary but efficient". The pianos built by Backers did not survive, but his action, patented in 1776, was refined by Robert Stodart (1777) and John Broadwood (1780); the resulting "English" action was noted by Hipkins as "the best single escapement action". Perhaps the best-known English piano action of the nineteenth century is the Brooks action of 1810.

Silbermann also was responsible for popularizing the Schröter or "German" action. Dolge characterizes Schröter's original action as "a model of innocent simplicity ... a clumsy device [that] made the touch hard and tough"; Silbermann improved the escapement of the original design. One of Silbermann's pupils, the Augsburg builder Johann Andreas Stein, further refined Schröter's action in the 1770s by reversing the orientation of the hammer, with the hammer head closer to the player. As described by Hipkins, in the original "Viennese" action, "the blow is caused by the depression of the key raising the hammer-butt until the back of it comes in contact with a rail at the back of the keyboard, the result being to jerk the hammer to the string". Stein also refined this action by adding an escapement. This "Viennese" action continued to be developed by Stein's daughter, Nannette Streicher, and was widely used by other makers in Vienna, and was the action of pianos played by Haydn, Mozart, and Beethoven, as it was characterized by a "pleasant light elastic touch and [a] charming musical quality of tone". It survived in Viennese pianos almost to the end of the 19th century.

In the 19th century, the English action was further modified by French builders, notably in the invention of the repetition lever, which facilitated rapidly repeating notes. Sébastien Érard invented the double-escapement action which incorporated the repetition lever, patented in 1808 and 1821; Dolge credits Érard with combining the refined touch of the Vienna action with the force and power of the English action. The 2000s-era grand piano action is a distant descendant of Cristofori's original.