The motet is the most important form of early polyphonic music, particularly during the Middle Ages and Renaissance.  Since it underwent numerous changes during the more than five centuries of its existence (c. 1220-1750), it is impossible to formulate a general definition that covers all the phases of its development.  As a rule, a motet is an unaccompanied choral composition based on a Latin sacred text and designed to be performed in the Roman Catholic service, chiefly at Vespers.  There are, however, side developments into the secular field (13th-cent. French motet; 15th-cent. ceremonial motet), as well as motets for soloists (13th-cent. motet; also in the late 17th cent.) or with orchestral accompaniment and to texts in the vernacular (17th cent., Germany and France; the English motets of the period are called anthems).  The history of the motet may be divided into three periods:

Medieval motet

The medieval motet originated in the early 13th century, possibly as early as 1200, through the addition of a full text to the upper (duplum) of the clausulae of the Perotin period, a development strikingly similar to the one that, in the 9th century, led from the vocalized melismas of the Alleluias to the full-text sequences.  Owing to the addition of "mots," the duplum with text was called "motetus," a name that was adopted for the entire composition.  The tenor of a motet (like that of a clausulae) is practically always a melismatic (vocalized) passage taken from Gregorian chant (usually a Gradual, Alleluia, or responsory) and identified by the word or syllable ("incipit", usually capitalized in modern writings) with which it occurs in the original plainsong.  The only change is in the rhythm, from the free rhythm of plainsong to a strict modal pattern.  It is important to realize that originally a motet was not an independent composition but, like the clausulae, a polyphonic interpolation (polyphonic trope) of the chant to which it is allied by its tenor.

The first step in the development of the 13th-century motet probably was to supply clausulae with a text that represents a paraphrase of the tenor incipit.  Frequently, however, not only unrelated Latin texts but also French secular texts were employed.  An important step was the addition of a third voice-part (triplum), in either Latin or French (a Latin triplum was used only in connection with a Latin motetus).

The rhythmic structure of the motet is based on the rhythmic modes, the upper parts frequently employing a quicker pattern (first, second, sixth mode) than the tenor (third, fifth mode).
In the 14th century the motet lost its dominant position but grew in length, elaboration, and rhythmic variety.  A feature of special interest is the introduction of the isorhythmic principle.  Practically all the motets of Machaut are isorhythmic, and a number of them apply this principle not only to the tenor but also, with a certain amount of freedom, to the upper parts.
Beginning in the 15th century, novel methods of composition were used for motets.  The two characteristics of the medieval motet, polytextuality and a cantus firmus tenor, were abandoned in favor of free composition with the same text in all the parts.  In the works of Dunstable and Dufay, free motets appear side by side with the last representatives of the isorhythmic type.  They are usually in three voice-parts and in cantilena style, with only the upper part (sometimes the two upper parts) carrying the text.

Flemish motet

The return to sacred music that characterizes the Flemish school brought the motet back to prominence as a musical form second in importance only to the Mass.  The motet now became a choral setting of a Latin religious text, in four to six or more voice-parts.  Its texture became much more unified, all the parts being vocal and having approximately the same degree of rhythmic animation.  Often, one part (usually the tenor) is made to stand out from the others by having a cantus firmus in slower motion.  Such cantus firmus motets, as they might be called, often have a different text for the main voice, a striking revival of the polytextuality of the 13th-century motet.  Others are often based on a short motif that is repeated in the manner of an ostinato.

The stylistic development of the Flemish motet is of great importance, because from c. 1450 to 1550 the motet provided the most fertile soil for all developments and innovations in style.  The most interesting aspect is the ever-increasing use of imitation, a process that culminated in the through-imitative style (also called "pervading imitation").  This style has been so much identified with the motet that it is often referred to as "motet style."  The motets of the 15h century (Ockeghem, Obrecht) make only sporadic use of imitation.  Josquin often introduced full points of imitation but in alternation with section in homophonic style, in free counterpoint, and in paired imitation.  It was Nicolas Gombert who introduced fully imitative treatment in the motet.

About 1550 the motet spread throughout Europe, and the Flemish masters (Josquin, Gombert, Lasso) found disciples of equal rank in Italy (A. Gabrieli, Palestrina, G. Gabrieli), Spain (Morales, Victoria), England (Tallis, Byrd), Germany (Senfl, Handl, Hassler), and France (Goudimel).  In England the adoption, c. 1560, of texts in the vernacular led to a special type of motet, the anthem.

Baroque motet

After 1600 the style of the motet changed considerably.  The pure a cappella style was abandoned, and solo voices as well as instrumental accompaniment were used.  This does not mean that the 16th-century style was completely forsaken.  Both the stile antico of Palestrina and the Venetian style with its massive sound were continuously cultivated in the motets of the Baroque, sometimes in almost unchanged manner.  Usually, however, the old methods were modified according to the stylistic devices of the 17th century, such as instrumental participation, solo voices, aria style, basso continuo, recitativo, etc.  The solo motet for two or three singers with organ accompaniment prevailed in Italy throughout the Baroque era, side by side with the choral style of Roman or Venetian tradition; several motets of the period made use of both styles.

The development in Germany can be marked as beginning with Schutz and ending with J. S. Bach.  Most of these pieces, particularly those from the later collections, are written to German texts.  It is difficult to draw a line between the German motet and other types of church music, such as the cantata and spiritual song.  As a rule, the use of the chorus marks the German form, since in Germany (unlike Italy) the motet remained choral and frequently a cappella.  The German motet reached its peak in the six motets by J. S. Bach, four of which are written for double chorus of eight voices.  After J. S. Bach, the motet declined.