A cloud chamber, also known as a Wilson chamber, is a particle detector used for visualizing the passage of ionizing radiation.

A cloud chamber consists of a sealed environment containing a supersaturated vapor of water or alcohol. An energetic charged particle (for example, an alpha or beta particle) interacts with the gaseous mixture by knocking electrons off gas molecules via electrostatic forces during collisions, resulting in a trail of ionized gas particles. The resulting ions act as condensation centers around which a mist-like trail of small droplets form if the gas mixture is at the point of condensation. These droplets are visible as a "cloud" track that persists for several seconds while the droplets fall through the vapor. These tracks have characteristic shapes. For example, an alpha particle track is thick and straight, while a beta particle track is wispy and shows more evidence of deflections by collisions.

Cloud chambers were invented in the early 1900s by the Scottish physicist Charles Thomson Rees Wilson. He played a prominent role in experimental particle physics from the 1920s to the 1950s, until the advent of the bubble chamber. In particular, the discoveries of the positron in 1932 (see Fig. 1) and the muon in 1936, both by Carl Anderson (awarded a Nobel Prize in Physics in 1936), used cloud chambers. The discovery of the kaon by George Rochester and Clifford Charles Butler in 1947 was made using a cloud chamber as the detector. In each of these cases, cosmic rays were the source of ionizing radiation. Yet they were also used with artificial sources of particles, for example in radiography applications as part of the Manhattan Project.

Charles Thomson Rees Wilson (1869–1959), a Scottish physicist, is credited with inventing the cloud chamber. Inspired by sightings of the Brocken spectre while working on the summit of Ben Nevis in 1894, he began to develop expansion chambers for studying cloud formation and optical phenomena in moist air. Very rapidly he discovered that ions could act as centers for water droplet formation in such chambers. He pursued the application of this discovery and perfected the first cloud chamber in 1911.

In Wilson's original chamber, the air inside the sealed device was saturated with water vapor, then a diaphragm was used to expand the air inside the chamber (adiabatic expansion), cooling the air and starting to condense water vapor. Hence the name expansion cloud chamber is used. When an ionizing particle passes through the chamber, water vapor condenses on the resulting ions and the trail of the particle is visible in the vapor cloud. A cine film was used to record the images.

Further developments were made by Patrick Blackett who utilised a stiff spring to expand and compress the chamber very rapidly, making the chamber sensitive to particles several times a second. This kind of chamber is also called a pulsed chamber because the conditions for operation are not continuously maintained.

Wilson received half the Nobel Prize in Physics in 1927 for his work on the cloud chamber (the same year as Arthur Compton received half the prize for the Compton Effect).

The diffusion cloud chamber was developed in 1936 by Alexander Langsdorf. This chamber differs from the expansion cloud chamber in that it is continuously sensitized to radiation, and in that the bottom must be cooled to a rather low temperature, generally colder than −26 °C (−15 °F). Instead of water vapor, alcohol is used because of its lower freezing point. Cloud chambers cooled by dry ice or Peltier effect thermoelectric cooling are common demonstration and hobbyist devices; the alcohol used in them is commonly isopropyl alcohol or methylated spirit.