Greer Garson as Marie Curie

Walter Pidgeon as Pierre Curie

Henry Travers as Eugene Curie

Albert Bassermann as Professor Jean Perot

Robert Walker as David Le Gros

C. Aubrey Smith as Lord Kelvin

May Whitty as Madame Eugene Curie

Victor Francen as President of University

Elsa Bassermann as Madame Perot

Reginald Owen as Dr. Becquerel

Van Johnson as Reporter

Margaret O'Brien as Irene Curie - Age 5

James Hilton as Narration Spoken By (voice)

Mariska Aldrich as Tall Woman (uncredited)

Tony Carson as Man at Accident (uncredited)

Ruth Cherrington as Swedish Queen (uncredited)

Ray Collins as Lecturer (voice) (uncredited)

Guy D'Ennery as Professor (uncredited)

George Davis as Cart Driver (uncredited)

Harold De Becker as Professor (uncredited)

Franz Dorfler as Assistant Tailor (uncredited)

Justine Duney as Woman at Accident (uncredited)

Tay Dunn as Man at Accident (uncredited)

William Edmunds as Cart Driver (uncredited)

Nestor Eristoff as Board Member (uncredited)

Al Ferguson as Man at Accident (uncredited)

Edward Fielding as Board Member (uncredited)

Howard Freeman as Prof. Constant (voice) (uncredited)

Linda Lee Gates as Perot Granddaughter (uncredited)

Marie Louise Gates as Perot Granddaughter (uncredited)

Lisa Golm as Lucille (uncredited)

Ilka Grüning as Seamstress (uncredited)

Lumsden Hare as Professor Roget (uncredited)

Teddy Infuhr as Son (uncredited)

James Kirkwood as Board Member (uncredited)

Isabel La Mal as Woman at Accident (uncredited)

Gene Lockhart as Bit Part (uncredited)

Miles Mander as Businessman (uncredited)

George Meader as Singing Professor (uncredited)

Dickie Meyers as Master Michaud (uncredited)

Noel Mills as Wedding Guest (uncredited)

Leo Mostovoy as Photographer (uncredited)

Alan Napier as Dr. Bladh (uncredited)

Moroni Olsen as President of Businessman's Board (uncredited)

María Pagés as Woman at Accident (uncredited)

Gigi Perreau as Eve at 18 Months (uncredited)

Francis Pierlot as Monsieur Michaud (uncredited)

Nita Pike as Woman at Accident (uncredited)

Dorothy Raye as Nurse (uncredited)

Almira Sessions as Madame Michaud (uncredited)

Arthur Shields as Businessman (uncredited)

Wyndham Standing as King Oscar (uncredited)

Ray Teal as Driver (uncredited)

Charles Trowbridge as Board Member (uncredited)

Michael Visaroff as Proud Papa (uncredited)

Marek Windheim as Jewelry Salesman (uncredited)

Frederick Worlock as Businessman (uncredited)

Eustace Wyatt as Doctor (uncredited)