A chimney is a system for venting hot gases and smoke from a stove, furnace or fireplace to the outside atmosphere. They are typically almost vertical to ensure the hot gases flow smoothly, drawing air into the combustion through convection. Chimneys may be found in buildings and steam locomotives and ships (for the latter, the US term is smokestack)

Chimneys have traditionally been built of brick, both in small and large buildings. Early chimneys were of a simple brick construction. Later chimneys were constructed by placing the bricks around tile liners. To control downdrafts venting caps with a variety of designs are sometimes placed on the top of chimneys. Due to brick's limited ability to handle traverse loads, chimneys in houses were often build in a "stack", with a fireplace on each floor of the house sharing a single chimney, often with such a stack at the front and back of the house. Today's central heating systems have made chimney placement less critical, and the use of non-structural double-wall metal piping allows it to be bent around obstructions and through walls. In fact, modern high-efficiency furnaces do not require a chimney and can vent sideways through a wall.

Masonry (brick) chimneys have proved particularly susceptible to crumbling during earthquakes. Government housing authorities in quake-prone cities like San Francisco and Los Angeles now recommend building new homes with stud-framed chimneys around a metal flue . (Bracing or strapping old masonry chimneys has not proved to be very effective in preventing damage or injury from earthquakes.) Perhaps predictably, a new industry provides "faux-brick" facades to cover these modern chimney structures.

Industrial chimneys were typically external structures, as opposed to being built into the wall of a building. Most often they were located near a central boiler, and the gases carried to it with external ductwork. Today the use of single-pour concrete has almost entirely replaced brick in this role. They can be quite tall. The height is to ensure the pollutants are dispersed over a wider area to meet legislative or safety requirements.

Trbovlje Chimney, 360 m high, serving Trbovlje coal power station

A characteristic problem with chimneys is they develop deposits of creosote on the walls of the structure when used with wood as a fuel. Deposits of this substance can interfere with the airflow and more importantly, they are flammable and can cause dangerous chimney fires if the deposits ignite in the chimney. Thus, in the United States it is recommended -- and in many other countries even mandatory -- that chimneys be inspected annually and cleaned on a regular basis to prevent these problems. The workers who perform this task professionally are called chimney sweeps.

Other problems include "spalling" brick, in which moisture seeps into the brick and then freezes, cracking and flaking the brick and loosening mortar seals.

An exhaust pipe serves a similar function to a chimney in moving machinery.

The term chimney may also be applied to natural features, particularly in rock formations.