Planetary atmospheres provide important records of planetary formation and evolution through their isotopic ratios, although these may require significant interpretation to provide scientific answers. Atmospheres are thought to arise in one of two ways: (i) primary, i.e. through original accretion, e.g. Jupiter, or (ii) secondary, having formed significantly later than the solid planet, either through external meteoroidal/cometary flux or via outgassing from the interior. The Earth’s atmosphere is postulated to be a member of this second category, forming after Moon Forming Event (MFE) and the end of the Late Heavy Bombardment (LHB) which would have eradicated any earlier atmosphere. Following formation, atmospheres subsequently evolve through loss processes including: sputtering (loss to space), photochemistry (conversion to other compounds) and interactions with the surface and even biosphere, such as carbonate rock formation, and other processes. Radiogenic (e.g. Argon-40, Carbon-14) and stable (non-radiogenic, e.g. Nitrogen-15, Carbon-13) isotopes can be measured through remote sensing and in situ sampling. ‘Stable’ isotopic ratios (e.g. C12/C13) are not unchanging over time, since loss processes can give preference to loss of one isotope over another, while ratios involving radiogenic isotopes (e.g. Ar-39/Ar-40) may also change over time, e.g. due to continued volcanic activity. The interpretation of these ratios therefore requires careful modeling and/or accurate assumptions about starting conditions. In this presentation I will summarize current knowledge of isotopic ratios in the dense planetary atmospheres of the solar system (Venus, Earth, Mars, the four giant/gas planets, and Titan) together with their current interpretations. I will also review the most important open questions and controversies, and areas where further and better measurements are most critically needed.