Available experimental data on chemical composition and crystal structure of K-bearing clinopyroxenes are compiled together with the results of atomistic simulations and thermodynamic calculations of mineral equilibria. It is shown that the limited solubility of K2O in clinopyroxene from crustal rocks cannot be ascribed to the strong non-ideality of mixing between diopside (CaMgSi2O6) and K-jadeite (KAlSi2O6) components. The more likely reason is the instability of the potassic endmember with respect to other K-bearing phases. As the volume effects of typical K-jadeite-forming reactions are negative, the incorporation of K in the clinopyroxene structure becomes less difficult at higher pressure. Atomistic simulations predict that the thermodynamic mixing properties of diopside–K-jadeite solid-solutions at high temperature approach those of a regular mixture with a relatively small positive excess enthalpy. The standard enthalpy of formation (ΔfH0 = −2932.7 kJ/mol), the standard volume (V0 = 6.479 J mol−1 bar−1) and the isothermal bulk modulus (K0 = 145 GPa) of K-jadeite were calculated from first principles, and the standard entropy (S0 = 141.24 J mol−1K−1) and thermal-expansion coefficient (α = 3.3 × 10−5 K−1) of the K-jadeite endmember were estimated using quasiharmonic lattice-dynamic calculations based on a force-field model. The estimated thermodynamic data are used to compute compositions of K-bearing clinopyroxenes in diverse mineral assemblages within a wide P-T interval. The review substantiates the conclusion that clinopyroxene can serve as an effective container for K at upper-mantle conditions.