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Ground object that impedes animals but allows humans and vehicles to pass
a device attached to the front of a locomotive

A cattle grid – also known as a stock grid in Australia; cattle guard, or cattle grate in American English; vehicle pass, or stock gap in the Southeastern United States; Texas gate in western Canada and the northwestern United States; and a cattle stop in New Zealand English – is a type of obstacle used to prevent livestock, such as sheep, cattle, pigs, horses, or mules from passing along a road or railway which penetrates the fencing surrounding an enclosed piece of land or border. It consists of a depression in the road covered by a transverse grid of bars or tubes, normally made of metal and firmly fixed to the ground on either side of the depression, so that the gaps between them are wide enough for an animal's feet to enter, but sufficiently narrow not to impede a wheeled vehicle or human foot. This provides an effective barrier to animals without impeding wheeled vehicles, as the animals are reluctant to walk on the grates.

The modern cattle grid for roads used by automobiles is said to have been independently invented a number of times on the Great Plains of the United States around 1905–1915. Before that period, a similar device for railroads was in use at least as early as 1836; a stone stile was used in Britain as far back as pre Roman times. An article in Texas Monthly claims that the "first recorded use of a cattle grid for nonrail traffic" occurred in 1881 in Archer County, Texas, on the stagecoach road between Archer City and Henrietta.

Cattle grids are usually installed on roads where they cross a fenceline, often at a boundary between public and private lands. They are an alternative to the erection of gates that would need to be opened and closed when a vehicle passes, and are common where roads cross open moorland, rangeland or common land maintained by grazing, but where segregation of fields is impractical. Cattle grids are also used when otherwise unfenced railways cross a fenceline. Cattle grids are common worldwide and are widespread in places such as Australia, the Scottish Highlands, or the National Parks of England and Wales. They are also common throughout the Western United States and Canada. In the United States, they are often used on Bureau of Land Management and Forest Service land, but are also used on paved roads and entry and exit ramps of the Interstate Highway System in rural areas.

Almost all cattle grids are built around a grid. Most include a pit dug along a fence line, a base for the grid to rest on, and wings to connect the guard to the fence. Since many guards were or are home made and to some degree idiosyncratic, a wide variety of designs exist. Designs may vary with local conditions. Materials used for construction of a cattle grid depend partly on the weight it must bear. A study of the bars of traditional cattle grids in the Flint Hills of Kansas found that 80 percent were made of pipe, while smaller percentages were made of railroad rails, I beams, planks, and other materials. The size of the bars varied from ; the spaces between bars varied from ; the number of bars per grid varied from 4 to 22. Grids differed in length from and in width from , while the pits beneath grids were deep.

Cattle grids made entirely or mostly of concrete have existed since the 1940s. Individual ranchers have often constructed their own, sometimes using plans developed in the 1940s. Commercial precast concrete versions are also available; Smith Cattleguard Company, based in Virginia, sold more than 15,000 of them between 1960 and 1980.

University Lands, which manages land and mineral interests for a foundation supporting the University of Texas and Texas A&M University systems, publishes do it yourself manuals for three sizes of cattle grids with grids made of steel pipes. The manuals include schematic drawings as well as accompanying instructions. Commercial guards made of steel are also available from multiple companies.

Painted lines on the road can serve as skeuomorphs of cattle grids. The light dark pattern of lines on pavement resembles a true cattle grid to animals, and by association think they will not be able to cross. Using a virtual cattle grid is initially cheaper than a true cattle grid, but the cost of periodic re painting may eventually exceed the initial cost of a well built metal guard. A particular advantage of painted guards is that they are smooth to drive over; in the US, most are found on state or federal highways, rather than private roads. Cattle may acquire the behavior of avoiding grids over pits either from individual experience or through imitation of other cattle. However, painted grids have been reported to work with semi wild cattle with no prior exposure to virtual grids. This is common; if one member of a herd discovers it can step safely on the lines, others will follow. Other incentives that lead cattle to test a virtual guard include placing food on the opposite side, or using strong driving pressure to run panicked cattle over a virtual grid. The primary advantage is cost and ease of installation. Drawbacks include the necessity of spraying vegetation with herbicides to keep weeds from shorting out the grid if there is no barrier between the wires and the ground. In addition, some low riding vehicles can catch the wires and tear them out. Another type was patented in two versions by an Oregon inventor in 1956–57; it consisted of 20 current carrying synthetic rubber strips mounted on a wooden frame. The invention proved highly effective in deterring all animals, including dogs, and it was maintenance free, easy to drive over, and safe. However, the company that acquired the manufacturing rights stopped production in 1960. Research has shown that deer can cross cattle grids with flat, as opposed to rounded bars. Sometimes a cattle grid is doubled to exclude these animals. A cattle grid requiring a horizontal leap of 14 feet (4 m) is considered effective when combined with a deer fence. Striping is also painted on roads as a visual deterrent to deer, as with "virtual" cattle grids.

While these barriers are usually effective for cattle, or run along the side of grids as wide as 8 feet (2 m). Wider grids are used where wildlife is to be contained. Some animals can jump across them, and a barrier that stops deer needs to be at least 16 feet (5 m) wide. In areas with heavy snowfall and long periods without a thaw, snow can accumulate beneath a grid and allow animals to walk across. Horses are particularly vulnerable to cattle grid injuries, as their single toed hooves can slip between the bars and trap their legs in an easily broken position. The same risk exists for kangaroos in Australia's outback, with additional risk of entrapment. Cattle grids are generally useless for containing goats. Cattle grids produce noise when vehicles pass over them and people living in a 100 m radius from the grid can be affected.

The United States Patent and Trademark Office (US PTO) issued patent on January 15, 1915, to William J. Hickey, Reno, Nevada, for a cattle grid. There is a British Standard for cattle grids: BS4008:2006. The US standards are put forth by The American Association of State Highway and Transportation Officials (AASHTO). AASHTO provides load rating guidelines for cattle grids that are used on public roads in the US. All cattle grids used on US public roads must be certified by a qualified engineer that the grid meets AASHTO guidelines.