The Blue Angel (German: Der blaue Engel) is a 1930 German musical comedy-drama film directed by Josef von Sternberg and starring Marlene Dietrich, Emil Jannings and Kurt Gerron.

Written by Carl Zuckmayer, Karl Vollmöller and Robert Liebmann, with uncredited contributions by Sternberg, it is based on Heinrich Mann's 1905 novel Professor Unrat (Professor Filth) and set in an unspecified northern German port city. The Blue Angel presents the tragic transformation of a respectable professor into a cabaret clown and his descent into madness. The film brought Dietrich international fame and introduced her signature song, Friedrich Hollaender and Robert Liebmann's "Falling in Love Again (Can't Help It)". The film is considered a classic of German cinema.

The film was shot simultaneously in German- and English-language versions. Though the English version was once considered a lost film, a print was discovered in a German film archive, restored and screened at San Francisco's Berlin and Beyond film festival on January 19, 2009. The German version is considered to be "obviously superior"; it is longer and not marred by actors struggling with English pronunciation.

Immanuel Rath is a professor at the local Gymnasium (high school for students expected to proceed to university) in Weimar Germany. The boys disrespect and play pranks on him. Rath punishes several of his students for circulating photographs of the beautiful Lola Lola, the headliner at the local cabaret called The Blue Angel. Hoping to catch the boys at the club, Rath goes there later that evening. He does find some students there, but while chasing them, he also finds Lola backstage and becomes infatuated with her. When he returns to the cabaret the following evening to return a pair of panties that was smuggled into his coat by one of his students, he winds up defending her from an aggressive patron. Lola is charmed by his chivalry, and serenades him from the stage. The next morning, Rath wakes up in Lola's bed. After having breakfast with Lola, Rath arrives late to school. Knowing of his visits to the Blue Angel, his pupils are openly mocking and contemptuous of him. Their misbehavior attracts a school official who, after dismissing the students, makes it clear to Rath that marrying a woman like Lola would end his career.

Rath resigns his position at the school and marries Lola. Their happiness is short-lived, however, as Rath becomes humiliatingly dependent on Lola. Over time, he sinks lower and lower, first selling dirty postcards, and then becoming a clown in Lola's troupe to pay the bills. His growing insecurities about Lola's profession as a "shared woman" eventually consume him with lust and jealousy as Lola and the rest of the troupe lose respect for him.

Five years after Rath's resignation from the school, the troupe returns to The Blue Angel, where everyone attends to watch the former professor play a clown. On stage, Rath is humiliated by the magician Kiepert, who breaks eggs on Rath's head, and by seeing Lola embrace and kiss the strongman Mazeppa. Rath is enraged to the point of insanity. He attempts to strangle Lola, but Mazeppa and others subdue him and lock him in a straitjacket.

Later that night, Rath is released and he revisits his old classroom. Rejected, humiliated and destitute, he dies clutching the desk at which he once taught.

The film also features the famous carillon of the Garrison Church at Potsdam playing "Üb' immer Treu und Redlichkeit" ("Always Be True and Faithful") as well as an orchestral version of the song. The original melody was composed by Wolfgang Amadeus Mozart for Papageno's aria "Ein Mädchen oder Weibchen" from The Magic Flute. In the German Reich and subsequently the Weimar Republic, the lyrics of "Üb' immer Treu und Redlichkeit" symbolized Prussian virtues: "Use always fidelity and honesty / Up to your cold grave / And stray not one inch / From the ways of the Lord".^{[citation needed]}