There is a deep historical relationship between thermodynamics and diamond synthesis. The success of high-pressure diamond synthesis can be regarded as a brilliant achievement under the theoretical guide of classical thermodynamics. Current nondissipative thermodynamics in modern thermodynamics are the direct result of the need to explain the new activated low-pressure diamond synthesis. The activated low-pressure diamond synthesis was first recognized by former Soviet scientists in about 1970, and made fun of as “alchemy” by some Western scientists at that time. Later, due to Japanese scientists’ work the new diamond technique spread into the West, and formed a research upsurge in the world in the latter 1980s and up to now. However, the thermodynamic explanation for the new technology is still based on our thermodynamic coupling model. An introduction of the thermodynamic coupling model and the problems and bepuzzlements associated with all other classical thermodynamics models will be discussed in this chapter. Note that any practical macroscopic processes including the new diamond process belong to dissipative thermodynamics, but all calculations of nonequilibrium phase diagrams for the low-pressure diamond synthesis belong to nondissipative thermodynamics, which will be mainly discussed in the following chapters, and this Chapter will be a link or transition from dissipative thermodynamics to nondissipative thermodynamics.