Achondrites are a type of stony meteorite that lack chondrules. Chondrules are the small, round inclusions seen in most stony meteorites in the chondrite family. These meteorites formed through a melting and recrystallization processes on their parent bodies, such as asteroids, the Moon, or Mars. This makes them significantly more 'evolved' compared to chondrites, which are considered 'primitive'.

Achondrites come from differentiated celestial bodies, meaning their parent bodies experienced enough internal heat to separate into distinct layers, with heavy metals forming a core and lighter silicate materials forming a crust and mantle. Achondrites represent materials from the outer layers of these differentiated bodies, often resembling volcanic or igneous rocks found on Earth. They formed as molten rock cooled and solidified, erasing the original chondrule structures that would have existed before melting occurred. As a result, these meteorites provide valuable information about planetary processes such as volcanism, impacts, and crust formation.

There are several important subgroups of achondrites, each associated with specific parent bodies. HED meteorites (howardites, eucrites, and diogenites) are believed to come from Asteroid 4 Vesta. Lunar meteorites come from the Moon, and Martian meteorites originate from Mars. Each of these subtypes offers crucial insights into the geological history of their parent body, including volcanic activity, surface impacts, and the internal structure of these objects.

Whilst achondrites make up only about 8% of all known meteorites, they are particularly important for planetary science. Because they come from bodies that have undergone differentiation, achondrites help researchers understand the processes that shaped not only these individual objects but also the larger mechanisms of planetary formation and evolution in the early solar system. Their study helps scientists piece together the history of planets and large asteroids, providing clues to their structure and development.