In music, harmony is the process by which individual sounds are joined or composed into whole units or compositions. Often, the term harmony refers to simultaneously occurring frequencies, pitches (tones, notes), or chords. However, harmony is generally understood to involve both vertical harmony (chords) and horizontal harmony (melody).

Harmony is a perceptual property of music, and, along with melody, one of the building blocks of Western music. Its perception is based on consonance, a concept whose definition has changed various times throughout Western music. In a physiological approach, consonance is a continuous variable. Consonant pitch relationships are described as sounding more pleasant, euphonious, and beautiful than dissonant relationships which sound unpleasant, discordant, or rough.

The study of harmony involves chords and their construction and chord progressions and the principles of connection that govern them.

Counterpoint, which refers to the relationship between melodic lines, and polyphony, which refers to the simultaneous sounding of separate independent voices, are therefore sometimes distinguished from harmony.

In popular and jazz harmony, chords are named by their root plus various terms and characters indicating their qualities. In many types of music, notably baroque, romantic, modern, and jazz, chords are often augmented with "tensions". A tension is an additional chord member that creates a relatively dissonant interval in relation to the bass.

Typically, in the classical common practice period a dissonant chord (chord with tension) "resolves" to a consonant chord. Harmonization usually sounds pleasant to the ear when there is a balance between consonance and dissonance. Simply put, this occurs when there is a balance between "tense" and "relaxed" moments. Dissonance is an important part of harmony when dissonance can be resolved and contribute to the composition of music as a whole. A misplayed note or any sound that is judged to detract from the whole composition can be described as disharmonious rather than dissonant.