This handbook concerned with the ternary phase diagram (polymer (p)/solvent (s)/nonsolvent (ns))and the evaluations of membrane-forming system. According to the theory of phase separation three modes of phase separation can take place in such ternary system: nucleation and growth of the polymer lean phase, spinodal phase separation and nucleation and growth of the polymer rich phase. Since polymer is one of the components of the ternary system, solidification of a part of the system can take place.The ternary phase diagram systems has an importand role to report the agreement between experimental work to get the required membranes, and the ternary phase diagram miscibility gaps for the evaluations of a membrane-forming system. In this book a general introduction on membrane science and technology is given. It begins with the definition of terms and provides a description of membranes structures and membrane separation process. The advantages as well as the limitations of membrane process are indicated. A short overview over the developments of membranes science and technology is given.