Comets appear to be a common phenomenon not just in our Solar System, but also in other planetary systems, both on the main sequence and around white dwarfs. The active cometary phase is transient, and to bring it about a body must be moved dynamically from its orbit in a source reservoir with a large periapsis to an orbit with a smaller periapsis. While these dynamical processes are well understood in the Solar System, with different dynamical channels giving rise to different families of comets, when it comes to most extrasolar systems we must fall back on speculation, as the planets that might be responsible for driving the dynamics are mostly undetectable.

A range of dynamical processes may contribute in different measure, and each system is likely to be unique in its details. These processes include Galactic tides, stellar flybys, direct gravitational scattering by planets, eccentricity forcing in mean-motion and secular resonances, and the von Zeipel–Lidov–Kozai effect induced by an inclined planet or binary stellar companion; multiple processes may be active in any one system, whether on different bodies or even the same body (for example, planetary scattering following resonant eccentricity forcing). Many of these mechanisms can not only deliver bodies to the inner regions of a planetary system to become comets, but also liberate them from the system entirely to become free-floating interstellar objects such as 1I/‘Oumuamua, 2I/Borisov, or 3I/ATLAS. In contrast to our lack of precise knowledge about the planetary demographics perturbing cometary source reservoirs, we have a reasonably good understanding of the demographics and evolution of massive Kuiper Belt analogues, which are visible in the infrared and submillimetre as debris discs. These are seen around main sequence stars of a range of spectral classes. A large number of debris discs have now been spatially resolved, especially with Herschel in the far-IR and ALMA in the sub-mm, and in many cases their morphology suggests ongoing dynamical interactions with one or more planets in the system, even though these have not yet been directly detected. For our understanding of the evolution of the cometary nuclei themselves, we are largely reliant on Solar System observations and extrapolation of these models to exocomets. Non-gravitational forces, arising from sublimation and irradiation near perihelion, have been directly observed to change the spins and orbits of small bodies in the Solar System. In our Solar System, we can observe cometary bodies at all stages of their evolution, from storage in the outer Solar System through to activity as they warm up when approaching the Sun, followed by eventual break-up, and with a number of comets now visited it is clear that the geology of these bodies can differ greatly, likely at least in part depending on how long they have been on their lowperihelion orbits. In the near future, it is hoped that the Comet Interceptor mission will be able to opportunistically study up close a dynamically new comet on its first perihelion passage through the inner Solar System, thus investigating a more pristine object. Our sample of ISOs is so far small, but has thrown up one major surprise in the unusual, possibly highly N2-rich, body 1I/‘Oumuamua.
Finally, a large number of white dwarfs show comet-like phenomena such as asymmetric transits. Deposition of material into the WD atmosphere provides an important means of probing the bulk composition of these objects, complementing other means of studying the compositions of extra-solar bodies such as planetary transmission spectroscopy. The material deposited into WD atmospheres appears to be largely depleted in volatiles, but it is as yet unclear whether these are on the whole actually volatilepoor asteroids, or formerly icy bodies more like Trans-Neptunian Objects that have been devolatilised by the extreme luminosity increase that their host star underwent during its giant branch evolution.