Having served as the music of the Roman Catholic Church for hundreds of years, chant has been the subject of improvisations throughout many different stylistic periods and in many different forms.
Some popular chant themes include:

• Agnus Dei XVIII
• Alma Redemptoris Mater
• Asperges me
• Attende Domine
• Ave Maris Stella
• Conditor Alme siderum
• Dies Irae
• In Paradisum
• Jesu dulcis memoria
• Jesu Redemptor gentium
• Kyrie Orbis Factor
• O Filii et Filiae
• Pange lingua gloriosi
• Puer Natus Est
• Regina Caeli
• Salve Regina
• Ubi caritas
• Veni Creator Spiritus
• Veni redemptor gentium
• Veni Sancte Spiritus
• Victimae paschali laudes

Videos:
Gerre Hancock – Improvised versets on the Magnificat Solemn Tone – April 4, 2004 – St. Thomas
Otto Maria Krämer – Improvisation in Memoriam Marcel Dupré on “Ave maris stella”
Loïc Mallié Improvisation sur deux thèmes grégoriens
Olivier Messiaen – Puer Natus Est – La Trinité
Pierre Pincemaille – Conditor Alme Siderum – St. Denis
William Porter – Improvisation: Four Modal Variations on Salve Regina: I (Theme and Plein jeu)
William Porter – Improvisation: Four Modal Variations on Salve Regina: II (Scherzo)
William Porter – Improvisation: Four Modal Variations on Salve Regina: III (Meditation)
William Porter – Improvisation: Four Modal Variations on Salve Regina: IV (Introduction and Passacaglia)