This thesis is an analytical study of various cadenzas written for the first movement of Mozart’s Piano Concerto No.20 in D minor, K.466. As one of the six of his own concertos for which Mozart did not provide an original cadenza, the D minor concerto poses an important challenge to the performer: should she compose or improvise her own cadenza, or should she select one written by someone else? Many composer/pianists active during the nineteenth and twentieth centuries penned cadenzas to this concerto for their own use, and this thesis explores those by August Eberhard Müller, Emanuel Aloys Förster, Ludwig van Beethoven, Johann Nepomuk Hummel, Charles-Valentin Alkan, Clara Schumann, Johannes Brahms, Ferruccio Busoni, Bedrich Smetana and Paul Badura-Skoda. In addition to these written-out cadenzas, it also discusses improvised cadenzas in the recordings by Robert Levin and Chick Corea. Each composer/pianist’s unique compositional style is illuminated through the study of each cadenza, and consideration of these styles allows multiple views on a single concerto. A discussion of the meaning and history of cadenzas precedes the analytical study, and in conclusion, the author contributes her own cadenza.