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The historical beginnings of the 18th-century fugue may be found in the 16th-century imitative ricercare (Italian: to search out). Illustrative examples are among works by Girolamo Cavazzoni, Andrea Gabrieli, and Jan P. Sweelinck in the 16th century, and Girolamo Frescobaldi in the 17th. It is a multi-thematic, contrapuntal piece featuring imitation--a sort of instrumental motet, written either for organ or a small ensemble. Garieli's ricercare apply various contrapuntal techniques that still appear in the Baroque (augmentation, diminution, stretto, inversion, invertible counterpoint).
As the genre evolved toward what we now know as fugue, the number of themes was reduced to one (sometimes two, occasionally three), and the interval of imitation in the first section limited to the fifth, where in the ricercare it had been variable. Further, the non-imitative sections of the ricercare disappeared so that, in the end, the fugue became a thoroughly imitative, thematically tightly knit (and hence highly unified) work.
Many 18th-century composers wrote fugues; it was the compositional workshop and proving ground of an aspiring composer. Johann Sebastian Bach's fugues, especially those in the Well-temepered Clavier and Art of the Fugue, are recognized as the pinnacle of the genre.