In this chapter, we review phase diagrams of alkali and alkaline earth carbonates at high pressures, particularly simple, binary, and ternary systems, which were recently constrained at pressures of 3 and 6 GPa. These studies revealed a number of new alkali-alkaline earth double carbonates. Major transformations of high-pressure carbonates, including changes in carbon coordination, spin transition, and valence state in Fe-bearing -carbonates up to the lower mantle levels, were also discussed. We emphasize the importance of carbonate systems for understanding the low-degree partial melting of carbonated mantle rocks and explaining carbonate inclusions in diamond and other deep-seated minerals. The question of carbonate stability versus the presumably reduced nature of the deep Earth's mantle provides significant impact on the further study of material transport and deep volatile cycle through the history of our planet.