It is unlikely that a definitive list of Widor's works can ever be established. He did not keep a complete or accurate record of his works and their dates of composition or revision; this is affirmed by a notation in his “Souvenirs autobiographiques,” p. 61: “Ask my publishers: Hamelle, Heugel, Durand, Schott, for the titles and dates of my different works.” Few autograph manuscripts have been discovered, and published scores (sometimes having multiple arrangements) are not always easily located.

Known opus numbers are taken either directly from a score or from publishers’ listings of Widor's works. Arrangements made by Widor or others during his lifetime are indicated, as known. Eight opus numbers are not filled and many works do not carry an opus number at all. An opus number may be found serving two works (indicated by a bracketed letter following the opus number), or a second opus number may serve a slightly revised work. The works designated by opus numbers are sometimes inexplicably out of chronological order. The dates given are the earliest discovered and are taken from the manuscript or score, calculated from publishers’ plate numbers, or taken from press reports.

Since Widor continually revised his music, oftentimes decades after the original date of composition, the works listed may have appeared in more than one version; this is not consistently indicated on the covers or by the plate numbers and can only be determined by painstaking comparisons made between several, apparently same, editions of a work. Because surviving copies rarely exist in the same location, such comparisons are burdensome at best. The author undertook this comparative process for the organ symphonies and determined eight different levels of revision for Opuses 13 and 42 during Widor's lifetime, with further emendations for a future printing indicated on his personal scores—see Charles-Marie Widor : The Symphonies for Organ, 10 vols.; vol. 11, “Preface,” contains a complete history of Opuses 13 and 42; vols. 19 and 20 have Opuses 70 and 73, respectively. Most of Widor's personal scores reside in the Bibliothèque nationale de France (Mus. Don 381–74) and carry many layers of corrections and emendations in his hand. Selected references to manuscripts in the Bibliothèque nationale are indicated as BN, and in the Bibliothèque de l'Opéra as BO.