In musical notation, an accidental is a symbol that indicates an alteration of a given pitch. The most common accidentals are the flat (♭) and the sharp (♯), which represent alterations of a semitone, and the natural (♮), which cancels a sharp or flat. Accidentals alter the pitch of individual scale tones in a given key signature; the sharps or flats in the key signature itself are not called accidentals.

An accidental applies to the note that immediately follows it and to subsequent instances of that note in the same measure, unless it is canceled by another accidental. A sharp raises a note's pitch by a semitone and a flat lowers it by a semitone. Double flats () or sharps () may also be used, altering the unmodified note by two semitones. If a note with an accidental is tied, the accidental continues to apply, even if the note it is tied to is in the next measure. If a note has an accidental and the note is repeated in a different octave, notation conventions vary. In modern notation, an accidental should be indicated on the second note to clarify, but some older notation assumes that the initial accidental applies across octaves.^{[citation needed]}

The modern accidental signs derive from the two forms of the lower-case letter b used in Gregorian chant manuscripts to signify the two pitches of B, the only note that could be altered. The "round" b became the flat sign, while the "square" b diverged into the sharp and natural signs.

In most cases a sharp raises the pitch of a note one semitone while a flat lowers it one semitone. A natural is used to cancel the effect of a flat or sharp. This system of accidentals operates in conjunction with the key signature, whose effect continues throughout an entire piece, or until another key signature is indicated. An accidental can also be used to cancel a previous accidental or reinstate the flats or sharps of the key signature.

Accidentals apply to subsequent notes on the same staff position for the remainder of the measure where they occur, unless explicitly changed by another accidental. Once a barline is passed, the effect of the accidental ends, except when a note affected by an accidental is tied to the same note across a barline. An accidental that carries past the barline through a tied note does not apply to subsequent notes.

Under this system, the notes in the example above are:

Though this convention is still in use particularly in tonal music, it may be cumbersome in music that features frequent accidentals, as is often the case in atonal music. As a result, an alternative system of note-for-note accidentals has been adopted, with the aim of reducing the number of accidentals required to notate a bar. According to Kurt Stone, the system is as follows:

Because seven of the twelve notes of the chromatic equal-tempered scale are naturals (the "white notes"; A, B, C, D, E, F and G on a piano keyboard) this system can significantly reduce the number of naturals required in a notated passage.