A mouse is a mammal that belongs to one of numerous species of small rodents in the genus Mus and various related genera of the family Muridæ (Old World Mice).

The best known mouse species is the common house mouse (Mus musculus). It is found in nearly all countries and, as the laboratory mouse, serves as an important model organism in biology; it is also a popular pet. (Non-biologists often use the term "mouse" synonymously with "Mus musculus"). The American white-footed mouse (Peromyscus leucopus) and the deer mouse (Peromyscus maniculatus) also sometimes live in houses. These species of mice live commensally with humans. Although they may live up to two years in the lab, the average mouse in the wild lives only 3 months, primarily due to heavy predation.

Mice are very common experimental animals in biology and psychology primarily because they are mammals, and thus share a high degree of homology with humans, but can be manipulated in ways that would be considered unethical to do with humans. Additional benefits include the fact that mice are small, relatively inexpensive, and several generations can be observed in a short period of time. The mouse genome has been sequenced, and many genes which share homology to human genes have been identified. In the 2006 Biosatellite project, a group of mice will orbit Earth inside a spinning spacecraft to determine how mice react to gravity equivalent to that of Mars.

However, mice can also be harmful pests, damaging and eating crops and spreading diseases through their parasites and feces. The domestication of cats is thought to have been for their predation of mice and their relatives, the rats. A mouse trap can also be used to catch mice.

Mice generally live on a herbivore diet, but are actually omnivores: they will eat meat, the dead bodies of other mice, and have been observed to cannibalise their tails during starvation.

An estimated half million mice live on the London Underground, mostly running around the tracks.

Mice cannot see colors, but they can see the difference between colors, because they see things in shade from black to white.