Pieing or a pie attack is the act of throwing a pie at a person. In Britain, a pie in the context of throwing is traditionally referred to as a custard pie. An aluminium pie pan or paper plate filled with whipped cream or more typically, shaving cream can substitute for a real pie, however, bakery pies such as chocolate cream pie, banana cream pie, coconut cream pie, or lemon meringue pie are also used, especially when one desires a more messy and humiliating effect. Brought to a widespread audience as the "pie-in-face" gag in silent film comedies, pieing may sometimes be intended as a harmless practical joke. However, it can also be used as a means of political protest directed against an authority figure, politician, industrialist, or celebrity, and perpetrators may regard the act as a form of ridicule.

Non-consensual pieing can constitute a punishable offence in criminal law (see, battery). Non-consensual pieing may also be actionable as a civil wrong (tort) giving the victim of the pieing the right to recover damages in a lawsuit.

Pieing and pie fights are a staple of slapstick comedy, and consensual pie "tosses" are also common charity fundraising events, especially in schools.

Pieing has its origins in the "pie in the face" gag from slapstick comedy. It made its first appearance in a music hall sketch called Mumming Birds (1904) produced by the English theatre impresario Fred Karno for the Hackney Empire in London. Immensely popular, it became the longest-running sketch the music halls produced. Charlie Chaplin and Stan Laurel, both employed by Karno, were among the music hall comedians who partook in the sketch, while Charlie's older brother Sydney was the first of the brothers to perform it for Karno.

It was first seen in film in the 1909 Essanay Studios silent film Mr. Flip starring Ben Turpin. In the story, Turpin has a pie pushed into his face for taking liberties with the woman behind the pie store counter. Beginning in 1913 with That Ragtime Band and A Noise from the Deep, filmmaker Mack Sennett became known for using one or two thrown pies in many of his comedy shorts. Sennett had a personal rule about who received the pies: "A mother never gets hit with a custard pie ... Mothers-in-law, yes. But mothers? Never."

In 1915, Chaplin's film A Night in the Show, which includes the pie in the face gag, brings one of the classic music hall comedy sketches, Mumming Birds, known as A Night in an English Music Hall when Chaplin performed it on tour, into his film work. At least a half dozen films have been made incorporating extended pie-throwing battles. The first was Chaplin's Behind the Screen released in 1916. The definitive pie fight in film occurs in The Battle of the Century (1927) starring Stan Laurel and Oliver Hardy, using 3,000 pies. Our Gang's Shivering Shakespeare (1930) winds up with an auditorium full of people throwing pies. The 1935 short subject Keystone Hotel featured a large pie-fight ending with the camera taking a pie. Another major pie-fight, their first, appeared in The Three Stooges' In the Sweet Pie and Pie (1941). Pieing had become such an established gag in Hollywood comedy that the song "Make 'Em Laugh" from Singin' In The Rain (1952) concludes with the line "And then you get a great big custard pie in the face!", the movie itself involves a pieing scene. A film involving pies was the comedy The Great Race (1965), known for having the largest pie fight in cinematic history. Its $200,000 pie-fight scene used 4,000 pies and one large cake, and took five days to shoot. Pie fights also featured in Beach Party (1963), Smashing Time (1967) and Blazing Saddles (1974). In Bugsy Malone (1976), the "splurge guns" resembled spud guns which fired custard. Original plans called for Dr. Strangelove (1964) to end with a pie fight; the scene, though filmed, was ultimately deemed excessively farcical by director Stanley Kubrick and removed from the final cut. Surviving stills from the excised pie fight have appeared online.

There are many instances in the Looney Tunes series of cartoons where characters pie each other in the face. Bugs Bunny repeatedly hits Elmer Fudd with cream pies during a scene in Slick Hare (1947), and also shoves one in Elmer's face in Hare Do. In Shishkabugs (1962), Bugs Bunny releases a spring-loaded pie into the face of the king, causing the royal cook Yosemite Sam to be led away to a dungeon. Daffy Dilly (1948) has Daffy Duck trying to cure a dying millionaire by getting him to laugh. After he achieves this inadvertently, by landing in a cake, Daffy is hired as a sort of household jester and ends the cartoon by getting repeatedly pelted with cakes and pies. Bugs himself gets pied in Case of the Missing Hare, provoking him to spend the rest of the short wreaking revenge.

Many comedy routines have used a pie as a gag, including ones performed by Soupy Sales and Monty Python, and those of clowns in many circus performances.