A grease trap (also known as a grease interceptor, grease recovery device, grease capsule, or grease converter) is a plumbing device (a type of trap) designed to intercept most greases and solids before they enter a wastewater disposal system. Common wastewater contains small amounts of oils which enter into septic tanks and treatment facilities to form a floating scum layer. This scum layer is very slowly digested and broken down by microorganisms in the anaerobic digestion process. Large amounts of oil from food preparation in restaurants can overwhelm a septic tank or treatment facility, causing the release of untreated sewage into the environment. High-viscosity fats and cooking grease such as lard solidify when cooled, and can combine with other disposed solids to block drain pipes.

Grease traps have been in use since the Victorian era; in the late 1800s, Nathaniel Whiting was granted the first patent. The quantity of fats, oils, greases, and solids (FOGS) that enter sewers is decreased by the traps. They consist of boxes within the drain run that flows between the sinks in a kitchen and the sewer system. They have only kitchen wastewater flowing through them and do not serve any other drainage system, such as toilets. They can be made from various materials, such as stainless steel, plastics, concrete and cast iron. They range from 35-liter capacity to 45,000 litres and greater. They can be located above ground, below ground, inside the kitchen, or outside the building.

There are three primary types of devices. The most common are those specified by American Society Of Mechanical Engineers (ASME), utilizing baffles, or a proprietary inlet diffuser.

Grease trap sizing is based on the size of the 2- or 3-compartment sink, dishwasher, pot sinks, and mop sinks. Many manufacturers and vendors offer online sizing tools to make these calculations easy. The cumulative flow rates of these devices, as well as overall grease retention capacity (in pounds or kilograms) are considered. Currently, ASME Standard (ASME A112.14.3) is being adopted by both of the national model plumbing codes (International Plumbing Code and Uniform Plumbing Code) that cover most of the US. This standard requires that grease interceptors remove a minimum of 90% of incoming FOGs. It also requires that grease interceptors are third-party tested and certified to 90 days compliance with the standard pumping. This third-party testing must be conducted by a recognized and approved testing laboratory.

Passive grease traps are generally smaller, point-of-use units used under three-compartment sinks or adjacent to dishwashers in kitchens.

Large in-ground tanks, usually 500–2,000 US gallons (2,000–8,000 L), are also passive grease interceptors. These units, made of concrete, fiberglass, or steel, have greater grease and solid storage capacities for high-flow applications such as a restaurant or hospital store. They are commonly called gravity interceptors. Interceptors require a retention time of 30 minutes to allow the fats, oils, grease, and food solids to settle in the tank. As more wastewater enters the tank, the grease-free water is pushed out of the tank. The rotting brown grease inside a grease trap or grease interceptor must be pumped out on a scheduled basis. The brown grease is not recycled and goes to landfills. On average 300 to 400 pounds (140 to 180 kg) of brown grease goes to landfill annually from each restaurant.

Passive grease traps and passive grease interceptors must be emptied and cleaned when 25% full. As the passive devices fill with fats, oils, and grease, they become less productive for grease recovery. A full grease trap no longer prevents FOG from entering the sanitary sewer system. The emptied contents or "brown grease" is considered hazardous waste in many jurisdictions.^{[citation needed]}

A third system type, hydromechanical grease interceptors (HGIs), has become more popular in recent years as restaurants open in more nontraditional sites. Often, these sites don't have space for a large concrete grease interceptor. HGIs take up less space and hold more grease as a percent of their liquid capacity — often between 70 and 85% of their liquid capacity or even higher as in the case of some "Trapzilla" models. These interceptors are 3rd-party certified to meet efficiency standards. Most are made out of durable plastic or fiberglass, lasting much longer than concrete gravity grease interceptors. They are usually lightweight and easy to install without heavy equipment. Most manufacturers test beyond the minimum standard to demonstrate the full capacity of the unit.