Terrestrial polar mesospheric clouds (PMCs or noctilucent clouds) are the highest clouds in the Earth’s atmosphere (83 km on average) and are composed of small water ice crystals. On Mars, mesospheric clouds have also been observed between altitudes of 50 and 100 km. Most observations suggest a CO₂ ice composition but water ice composition has also been reported. The mesospheric clouds on both planets form under the influence of gravity waves in an extremely tenuous part of the atmosphere probably on exogenous condensation nuclei (CN) provided by meteoroid ablation. Clouds composed of sulphuric acid solution droplets are observed in the atmosphere of Venus and in the Earth's stratosphere. The concentration of the Earth’s stratospheric sulphate aerosols varies strongly as a function of volcanic eruptions; these aerosols can serve as CN for the formation of polar stratospheric clouds that are catalysers of ozone destruction. The 20-km thick layer of Venus’ clouds, which is found at 50-70 km altitude and covers the whole planet, lets only a few per cent of solar radiation reach the surface and is an important sink of atmospheric sulphur. Both Venus’ clouds and Earth’s sulphate aerosol layer have a cooling effect on the climate and play an important role in atmospheric chemistry. We are currently developing a Venus cloud model based on an aerosol/cloud model for the Earth’s stratosphere.We will present an overview of these clouds and aerosols on the three planets through review of observations and modelling. We will discuss the properties of the clouds and recent results on their formation mechanisms, largely based on work carried out within our group.