A nose is a sensory organ and respiratory structure in vertebrates. It consists of a nasal cavity inside the head, and an external nose on the face. The external nose houses the nostrils, or nares, a pair of tubes providing airflow through the nose for respiration. Where the nostrils pass through the nasal cavity they widen, are known as nasal fossae, and contain turbinates and olfactory mucosa. The nasal cavity also connects to the paranasal sinuses (dead-end air cavities for pressure buffering and humidification). From the nasal cavity, the nostrils continue into the pharynx, a switch track valve connecting the respiratory and digestive systems.

In humans, the nose is located centrally on the face and serves as an alternative respiratory passage especially during suckling for infants. The protruding nose that is completely separate from the mouth part is a characteristic found only in therian mammals. It has been theorized that this unique mammalian nose evolved from the anterior part of the upper jaw of the reptilian-like ancestors (synapsids).

Acting as the first interface between the external environment and an animal's delicate internal lungs, a nose conditions incoming air, both as a function of thermal regulation and filtration during respiration, as well as enabling the sensory perception of smell.

Hair inside nostrils filter incoming air, as a first line of defense against dust particles, smoke, and other potential obstructions that would otherwise inhibit respiration, and as a kind of filter against airborne illness. In addition to acting as a filter, mucus produced within the nose supplements the body's effort to maintain temperature, as well as contributes moisture to integral components of the respiratory system. Capillary structures of the nose warm and humidify air entering the body; later, this role in retaining moisture enables conditions for alveoli to properly exchange O₂ for CO₂ (i.e., respiration) within the lungs. During exhalation, the capillaries then aid recovery of some moisture, mostly as a function of thermal regulation, again.

The wet nose of dogs is useful for the perception of direction. The sensitive cold receptors in the skin detect the place where the nose is cooled the most and this is the direction a particular smell that the animal just picked up comes from.

In amphibians and lungfish, the nostrils open into small sacs that, in turn, open into the forward roof of the mouth through the choanae. These sacs contain a small amount of olfactory epithelium, which, in the case of caecilians, also lines a number of neighbouring tentacles. Despite the general similarity in structure to those of amphibians, the nostrils of lungfish are not used in respiration, since these animals breathe through their mouths. Amphibians also have a vomeronasal organ, lined by olfactory epithelium, but, unlike those of amniotes, this is generally a simple sac that, except in salamanders, has little connection with the rest of the nasal system.

In reptiles, the nasal chamber is generally larger, with the choanae located much further back in the roof of the mouth. In crocodilians, the chamber is exceptionally long, helping the animal to breathe while partially submerged. The reptilian nasal chamber is divided into three parts: an anterior vestibule, the main olfactory chamber, and a posterior nasopharynx. The olfactory chamber is lined by olfactory epithelium on its upper surface and possesses a number of turbinates to increase the sensory area. The vomeronasal organ is well-developed in lizards and snakes, in which it no longer connects with the nasal cavity, opening directly into the roof of the mouth. It is smaller in turtles, in which it retains its original nasal connection, and is absent in adult crocodilians.

Birds have a similar nose to other reptiles, with the nostrils located at the upper rear part of the beak. Since they generally have a poor sense of smell, the olfactory chamber is small, although it does contain three turbinates, which sometimes have a complex structure similar to that of mammals. In many birds, including doves and fowls, the nostrils are covered by a horny protective shield. The vomeronasal organ of birds is either under-developed or altogether absent, depending on the species.