Millions of animals are used annually in research, testing, and education in the United States. These animals include (in decreasing order of frequency) mice, rats, birds, rabbits, guinea pigs, hamsters, "farm animals" such as pigs and sheep, dogs, primates, and cats. Frogs and fish are also widely used, but current usage statistics are unavailable.

Most animals who end up in laboratories are purpose-bred— that is, bred specifically for laboratory use. Others, however, including about 20% of primates used, are taken from the wild. Most of the frogs and dogfish procured for classroom dissection and vivisection come from the wild. About half of the dogs and cats used in laboratories are former pets who have been taken from animal shelters (a practice known as "pound seizure") or purchased from brokers who acquire the animals at auctions, from newspaper ads, or from various other sources. Fetal pigs used in dissections come from slaughterhouses.

Unfortunately, no accurate and comprehensive figures are available on how many animals are used—or for what purposes they are used. The U.S. Department of Agriculture does compile annual statistics on the use of dogs, cats, primates, rabbits, hamsters and guinea pigs (as well as some wild animals and, recently, farm animals), but the most common laboratory animals—rats and mice, which make up 85–90% of all animals used—are not counted. The Office of Technology Assessment estimated research animal use, for all species of (vertebrate) animals, to be 17 to 22 million animals annually during the mid-1980s (OTA, 1985).

As a result of new technology, pressure from the animal protection movement, new regulatory hurdles, and the rising cost of procuring and maintaining lab animals, the worldwide use of animals has declined substantially over the past 20 to 30 years. At the same time, interest in non-animal approaches is growing. There are many problems in trying to determine trends in lab animal use in the United States but in Europe, where the statistics are more detailed and the record-keeping better, the numbers show a 50% or larger decline in animal use in Great Britain, the Netherlands and Switzerland from the 1970s to the present. Other European countries also report substantial declines, and these data, coupled with data on the declining use of animals by the U.S. Department of Defense and by corporations, indicate that the same trends are operating in the United States.