The phenomena associated with phase separation via spinodal decomposition (SD) and nucleation of binary and ternary partially miscible solvent systems are reviewed. The pertinent literature includes many theoretical, numerical and experimental studies which were conducted in order to follow the flow phenomena during the phase separation of solutions of critical and off-critical compositions. The unique characteristics of phase transition in partially miscible solvent systems include efficient mass transfer in the single phase i-egion, rapid phase separation, low sensitivity to presence of emulsifiers and solids, high penetration of solvents to wetted pores, and convective motion of the separating droplets due to chemical potential gradients. These have beenutilized for the development of a novel extraction process, denoted as Phase Transition Extraction (PTE) for difficult tasks, and for enhancing convective heat transfer rates. Interesting aspects of the phase separation phenomena in these solvent systems and their wide range potential practical applications are demonstrated and discussed.