Western Ashkenazi cantors have developed a new soloist style of liturgically important melodies by inserting new and prolongued pieces into the traditional motivic structure. The "Fantasia" pieces found in manuscripts and printed scores from the period of 1736-1900 are: "Alenu LeSabbeah" for High Holidays' Musaf, "Avot" for HH Shaharit, "Al Harishonim" for the three Festivals, Hatzi Kaddish for HH Musaf, "Yaale Tahnunenu" for Kol Nidrei, and "VeHaCohanim" from the "Avoda" of Yom Kippur. The cantorial fantasia on "Alenu", based on a melody which is considered one of the MiSinai Tunes, is typical of this genre and is analyzed in the article through 8 different versions. The author describes the structure of the fantasia as a musical form and the compositional techniques involved in its creation. In addition, extra-musical elements, such as exegesis and deep meanings inherent to the prayer are examined to the extent they are relevant to the building of the composition. Eventually, the historical place of the cantorial fantasia and the factors leading to its decline in traditional litrugical practice are discussed.