Virtually unknown outside of Scandinavia until she was nearly 40 years old (and considering retirement), Kirsten Flagstad exploded upon the operatic firmament with a Metropolitan Opera debut in 1935 which happened to be broadcast across the United States and Canada. Overnight, she became the preeminent Wagnerian soprano of her generation. Some called Flagstad "the voice of the century.” She travelled West to San Francisco later that year to sing all three of Wagner's Brünnhildes for the first time, in the opera company's first Ring. In the next season, the San Francisco Opera succeeded in doing what the Metropolitan was never able to do: put the two leading Wagnerian sopranos of the time on stage simultaneously. In two performances of Die Walküre, audiences in the War Memorial Opera House heard Flagstad as Brünnhilde and Lotte Lehmann as Sieglinde, with Fritz Reiner conducting, Lauritz Melchior as Siegmund, Emanuel List as Hunding, and Friedrich Schorr as Wotan.

After virtually retiring in 1941 to be with her husband in Norway during the Second World War, Flagstad returned to the United States in 1949, where enjoyed great success in Tristan und Isolde and Walküre at the San Francisco Opera in the Fall of 1949 (at the age of 54). Following her death in 1962, her grandson divided her private tape collection between the cities of her greatest triumphs: New York and San Francisco. Thus the Archive at Stanford shares with the Rodgers and Hammerstein Archives of Recorded Sound of The New York Public Library this significant legacy of a singer whose career spanned the history of recording, from acoustic to stereo.

The Flagstad Collection contains recordings (86 sound tape reels) of Kirsten Flagstad and Waldemar Alme in performances and interviews.