A cheesesteak (also known as a Philadelphia cheesesteak, Philly cheesesteak, cheesesteak sandwich, cheese steak, or steak and cheese) is a sandwich made from thinly sliced pieces of beefsteak (often rib eye) and melted cheese in a long hoagie roll. A popular regional fast food, it has its roots in the United States city of Philadelphia, Pennsylvania.

The cheesesteak was developed in the early 20th century "by combining frizzled beef, onions, and cheese in a small loaf of bread," according to a 1987 exhibition catalog published by the Library Company of Philadelphia and the Historical Society of Pennsylvania.

Philadelphians Pat and Harry Olivieri are often credited with inventing the sandwich by serving chopped steak on an Italian roll in the early 1930s. The exact story behind its creation is debated, but according to many accounts, Pat and Harry Olivieri originally owned a hot dog stand, and on one occasion, decided to make a new sandwich using chopped beef and grilled onions. While Pat was eating the sandwich, a cab driver stopped by and was interested in it, so he requested one for himself. After eating it, the cab driver suggested that Olivieri quit making hot dogs and instead focus on the new sandwich. They began selling this variation of steak sandwiches at their hot dog stand near South Philadelphia's Italian Market. They became so popular that Pat opened up his own restaurant which still operates today as Pat's King of Steaks.

The sandwich was originally prepared without cheese. According to Pat's son Herb Olivieri, American cheese was first added by "Cocky Joe" Lorenza, a manager at the Ridge Avenue location in March of 1951. (However, other Olivieri members may say it was provolone.) There is some dispute over whether Pat's was the first to add cheese, though to date no concrete evidence has emerged to disprove Olivieri's story.

Cheesesteaks have become popular at restaurants and food carts throughout the city with many locations being independently owned, family-run businesses. Variations of cheesesteaks are now common in several fast food chains. Versions of the sandwich can also be found at high-end restaurants. Many establishments outside of Philadelphia refer to the sandwich as a "Philly cheesesteak".

The meat traditionally used is thinly sliced rib-eye or top round, although other cuts of beef are also used. On a lightly oiled griddle at medium temperature, the steak slices are quickly browned and then scrambled into smaller pieces with a flat spatula. Slices of cheese are then placed over the meat, letting it melt, and then the roll is placed on top of the cheese. The mixture is then scooped up with a spatula and pressed into the roll, which is then cut in half.

Common additions include sautéed onions, grilled mushrooms, ketchup, hot sauce, salt, and black pepper.

In Philadelphia, cheesesteaks are invariably served on hoagie rolls. Among several brands, perhaps the most famous are Amoroso rolls; these rolls are long, soft, and slightly salted. One source writes that "a proper cheesesteak consists of provolone or Cheez Whiz slathered on an Amoroso roll and stuffed with thinly shaved grilled meat," while a reader's letter to an Indianapolis magazine, lamenting the unavailability of good cheesesteaks, wrote that "the mention of the Amoroso roll brought tears to my eyes." After commenting on the debates over types of cheese and "chopped steak or sliced", Risk and Insurance magazine declared, "The only thing nearly everybody can agree on is that it all has to be piled onto a fresh, locally baked Amoroso roll."