My Opposition is a diary secretly written by the German social democrat Friedrich Kellner (1885–1970) during World War II to describe life under Nazi Germany and to expose the propaganda and the crimes of the Nazi dictatorship. Comprising ten notebooks, it is considered by leading historians as "an important piece of historical literature." The editors of the German magazine Der Spiegel called it "an image of Nazi Germany that has never existed before in such a vivid, concise and challenging form." Kellner began his 861-page diary on September 1, 1939, and wrote his last entry on May 17, 1945.

A diary is a written or audiovisual memorabilic record, with discrete entries arranged by date reporting on what has happened over the course of a day or other period. Diaries have traditionally been handwritten but are now also often digital. A personal diary may include a person's experiences, thoughts, and/or feelings, excluding comments on current events outside the writer's direct experience. Someone who keeps a diary is known as a diarist. Diaries undertaken for institutional purposes play a role in many aspects of human civilization, including government records, business ledgers, and military records. In British English, the word may also denote a preprinted journal format.