A pack rat or packrat, also called a woodrat or trade rat, are any species in the North and Central American rodent genus Neotoma. Pack rats have a rat-like appearance, with long tails, large ears, and large, black eyes. Pack rats are noticeably larger than deer mice, harvest mice, and grasshopper mice, and are usually somewhat larger than cotton rats.

Neotoma includes three subgenera (daggers (†) mark extinct species):

Packrats typically have wide eyes accompanied with long whiskers. They have large ears that extend outwards. In terms of size, they resemble Norway rats. They grow to be approximately 30 cm (12 in) long, with their tails making up about 50% of their size. Long-haired pack rats specifically have tails that are adequately furry and are physically compared to the tails of short-haired squirrels. In general, packrats' backs are notably a hybrid hue of brown and grey, but their underbelly tends to be a lighter shade.

Woodrats reach their greatest diversity in the deserts of the western United States and northern Mexico. Several species are also found in the deciduous forest of the east coast, juniper woodlands in the southwest, oak woodlands along the coastal western United States and in the Sonoran Desert, and in the forest and rocky habitats of the western United States and western Canada.

Each species of pack rat is generally restricted to a given type of habitat within its range. Pack rats live anywhere from low, hot, dry deserts to cold, rocky slopes above timberline. Pack rats build complex houses or dens made of twigs, cactus joints, and other materials. These contain several nest chambers, food caches, and debris piles. Dens are often built in small caves or rocky crevices, but when close by human habitations, woodrats will opportunistically move into the attics and walls of houses. Some Neotoma species, such as the white-throated woodrat (N. albigula), use the bases of prickly pear or cholla cactus as the sites for their homes, using the cactus' spines for protection from predators. Others, like the desert woodrat (N. lepida), will build dens around the base of a yucca or cactus, such as jumping and teddy-bear chollas. The largest species, Neotoma cinerea, has a bushy, almost squirrel-like tail. Bushy-tailed woodrats Neotoma cinerea occupy a range of habitats from boreal woodlands to deserts. They are cliff-dwellers and are often found on isolated, high-elevation exposed boulder areas under a variety of temperature and moisture conditions. They require adequate shelter among the rocks, though they are occasionally found inhabiting abandoned buildings as well.

Pack rats are nest builders. They use plant material such as twigs, sticks, and other available debris. They are particularly fond of shiny objects. A peculiar characteristic is that if they find something they want, they will drop what they are currently carrying—for example, a piece of cactus—and "trade" it for the new item. They can also be quite vocal and boisterous. They can easily become a nuisance by entering and inhabiting human spaces like attics or car engines, stealing their treasures from humans, damaging electrical wiring, and creating general noisy havoc.

Bushy-tailed woodrats feed primarily on green vegetation, twigs, and shoots. Mexican woodrats eat seeds, fruits, acorns, and cactus. The pack rat microbiome has symbiotic roles in digestion, recycling endogenous nitrogen, and the detoxification of dietary toxins. The bacterial composition of the pack rat's gut microbiome is affected by what it eats, and by association, the geography of where the animal is from. However, while the bacterial composition of the pack rat microbiome is influenced by diet and geography, it is the animals' genetics that has the most influential role on bacterial composition.

Adult bushy-tailed woodrat males usually weigh 300–600 g (11–21 oz), with an average of 405 g (14.3 oz), and adult females usually weigh 250–350 g (8.8–12.3 oz), with an average of 270 g (9.5 oz). These ranges are relatively large because this species occupies a large geographic range, and its body size is closely correlated with climate. Average males range in size from 310 to 470 mm (12 to 19 in), with the average being 379 mm (14.9 in), and average females range from 272 to 410 mm (10.7 to 16.1 in), with the average being 356 mm (14.0 in).