Heliostats are scientific instruments that track the progress of the sun across the heavens. Simple heliostats use an ordinary clockwork mechanism to follow the sun, while more complex heliostats track the sun directly by sensing its position in the sky or incorporate microprocessing software that compansates for the angle of the sun throughout the solar year. The word heliostat is a compound of the Greek word for 'sun', helios, and the word for 'still' or 'stationary', stat, and it appears to have been coined in the mid 18th century. As with many scientific inventions from that period, a number of inventors have been credited for the device, making it unclear who is actually responsible for it.

Heliostats are most commonly used to position mirrors and reflect sunlight continuously in a specific direction, although strictly speaking, heliostat refers to the positioning device and not the mirror. The most frequent use of heliostats in the 18th century, when they became widespread, was to position mirrors for astronomy observations and optical experiments. In the 20th century, arrays of mirrors positioned on heliostats were used in a range of mostly scientific applications, including electricity generation, astronomy, architecture, and solar lighting.

An organic example of the heliostat exists in the sunflower, which is known to track the movement of the sun across the sky to maximize exposure. Mechanical heliostats work in a similar way, and usually with the same end goal of harnessing the sun's energy. The most basic heliostat involves a pivoting clockwork mechanism with a mirror positioned on it to follow the sun's progress across the sky. In a laboratory setting, where the device can be moved to compensate for changes in the sun's angle, this type of device is adequate. More complex automated heliostats use software and sensors to orient themselves in relationship to the sun. Companies that sell heliostats to provide architectural accents or integral lighting solutions usually offer them with embedded microprocessors capable of calculating solar positioning algorithms.

Massive arrays of mirrors mounted on heliostats reflecting light to specific locations and distribution systems can be found in high rise buildings, on solar energy farms, as architectural effects on structures all over the world, and in many astronomy laboratories. Use of heliostats in architecture is growing, as sunlight is effective, free, and pleasing ambient lighting. In labs, heliostats are used for solar observations, including measurements of solar radiation, as well as for energy production, heat generation, and other applications. Many labs also have siderostats, designed for tracking stars other than the sun.