A diner is a type of restaurant found across the United States and Canada, as well as parts of Western Europe and Australia. Diners offer mostly American cuisine, a casual atmosphere, and, typically, a combination of booths served by a waitstaff and a long sit-down counter with direct service, in the smallest simply by a cook. Many diners have extended hours, and some along highways and areas with significant shift work stay open for 24 hours.

Considered quintessentially American, many diners share an archetypal exterior form. Some of the earliest were converted rail dining cars, retaining their streamlined structure and interior fittings. From the 1920s to the 1940s, diners, by then commonly known as "lunch cars", were usually prefabricated in factories, like modern mobile homes, and delivered on site with only the utilities needing to be connected. As a result, many early diners were typically small and narrow to fit onto a rail car or truck. This small footprint also allowed them to fit into tiny and relatively inexpensive lots that were unable to support a larger enterprise. Diners were historically small businesses operated by the owner, with some presence of restaurant chains evolving over time.

Diners typically serve staples of American cuisine such as hamburgers, hot dogs, club sandwiches, french fries, onion rings, and other simple, quickly cooked, and inexpensive fare, such as meatloaf or steak. Much of the food is grilled, as early diners were based around a gas-fueled flattop grill. Coffee is a diner staple. Diners often serve milkshakes and desserts such as pies, cake or ice cream. Comfort food cuisine draws heavily from, and is deeply rooted in, traditional diner fare. Along with greasy spoon menu items, many diners will serve regional cuisine as well, such as clam chowder in New England and tacos in California.

Classic American diners often have an exterior layer of stainless steel siding, a feature unique to diner architecture. In some cases, diners share nostalgic, retro-style features also found in some restored drive-ins and old movie theatres.

A crude precursor of the diner was created in 1872 by Walter Scott, who sold food out of a horse-pulled wagon to employees of the Providence Journal, in Providence, Rhode Island. Scott's diner can be considered the first diner with walk-up service, as it had windows on each side of the wagon.^{[citation needed]} Commercial production of such "lunch wagons" began in Worcester, Massachusetts, in 1887, by Thomas Buckley. Buckley was successful and became known for his "White House Cafe" wagons. Charles Palmer received the first patent (1893) for the diner, which he billed as a "Night-Lunch Wagon". He built his "fancy night cafes" and "night lunch wagons" in the Worcester area until 1901.

As the number of seats increased, wagons gave way to pre-fabricated buildings made by many of the same manufacturers which had made the wagons. Like the lunch wagon, a stationary diner allowed one to set up a food service business quickly using pre-assembled constructs and equipment.

The Transfer Station neighborhood of Union City, New Jersey was the site, in 1912, of the first lunch wagon built by Jerry and Daniel O'Mahoney and John Hanf, which was bought for $800 and operated by restaurant entrepreneur Michael Griffin, who chose the location for its copious foot traffic. The wagon helped spark New Jersey's golden age of diner manufacturing, which in turn made the state the diner capital of the world. In the decades that followed, nearly all major US diner manufacturers, including Jerry O'Mahoney Inc., started in New Jersey. Jerry O'Mahony (1890–1969), who hailed from Bayonne, New Jersey, is credited by some to have made the first such "diner". The O'Mahony Diner Company of Elizabeth, New Jersey, produced 2,000 diners from 1917 to 1952. Only approximately twenty remain throughout the United States and abroad. Others more credibly credit Philip H. Duprey and Grenville Stoddard, who established the Worcester Lunch Car and Carriage Manufacturing Company in Worcester, Massachusetts, in 1906, when O'Mahony was still just 16.

Until the Great Depression, most diner manufacturers and their customers were located in the Northeast. Diner manufacturing suffered with other industries during the Depression, though not as much as many industries, and the diner offered a less expensive way of getting into the restaurant business as well as less expensive food than more formal establishments. After World War II, as the economy returned to civilian production and the suburbs boomed, diners were an attractive small business opportunity. During this period, diners spread beyond their original urban and small town market to highway strips in the suburbs, even reaching the Midwest, with manufacturers such as Valentine. After the Interstate Highway System was implemented in the US in the 1960s, diners saw a boom in business as mobile travelers would stop for a meal.