A fire engine or fire truck (also spelled firetruck) is a vehicle, usually a specially designed or modified truck, that functions as a firefighting apparatus. The primary purposes of a fire engine include transporting firefighters and water to an incident as well as carrying equipment for firefighting operations in a fire drill. Some fire engines have specialized functions, such as wildfire suppression and aircraft rescue and firefighting, and may also carry equipment for technical rescue.

Many fire engines are based on a commercial vehicle chassis that is further upgraded and customized for firefighting requirements. They are generally considered emergency vehicles authorized to be equipped with emergency lights and sirens, as well as communication equipment such as two-way radios and mobile computer technology.

The terms fire engine and fire truck are often used interchangeably to a broad range of vehicles involved in firefighting; however, in some fire departments they refer to separate and specific types of vehicle.

The design and construction of fire engines focuses greatly on the use of both active and passive warnings. Passive visual warnings involve the use of high contrast patterns to increase the noticeability of the vehicle. They are more common on older vehicles or those in developing countries. Modern vehicles in developed countries use retroreflectors to create designs or display words such as "fire" or "rescue". European countries commonly use a pattern known as Battenburg markings. Along with the passive warnings, are active visual warnings which are usually in the form of flashing colored lights (also known as "beacons" or "lightbars"). These flash to attract the attention of other drivers as the fire truck approaches, or to provide warning to drivers approaching a parked fire truck in a dangerous position on the road. While the fire truck is headed towards the scene, the lights are always accompanied by loud audible warnings such as sirens and air horns. Some fire engines in the United States are lime yellow rather than red due to safety and ergonomics reasons. A 2009 study by the U.S. Fire Administration concluded that fluorescent colors, including yellow-green and orange, are easiest to spot in daylight.

In some regions, a fire engine may be used to transport first responder firefighters, paramedics or EMTs to medical emergencies due to their proximity to the incident.

The standard fire engine transports firefighters to the scene, carries equipment needed by the firefighters for most firefighting scenarios, and may provide a limited supply of water with which to fight the fire. The tools carried on the fire engine will vary greatly based on many factors including the size of the department and the usual situations the firefighters handle. For example, departments located near large bodies of water or rivers are likely to have some sort of water rescue equipment. Standard tools found on nearly all fire engines include ladders, hydraulic rescue tools (often referred to as the jaws of life), floodlights, fire hose, fire extinguishers, self-contained breathing apparatus, and thermal imaging cameras.

The exact layout of what is carried on an engine is decided by the needs of the department. For example, fire departments located in metropolitan areas will carry equipment to mitigate hazardous materials and effect technical rescues, while departments that operate in the wildland-urban interface will need the gear to deal with brush fires.

Fire engines may have a deluge gun, also called master streams, which directs a heavy stream of water to wherever the operator points it. Engines may also have a preconnected hose lines, known simply as preconnects, which remain attached to an onboard water supply as opposed to a .fire hydrants or water tenders.