The monograph summarizes and analyses the results of studies into phase transitions in polymer solutions, blends and gels induced by mechanical field (tension stress, compression, shear deformation, positive and negative external pressure), electric and magnetic fields and beyond the field for the last 60 years. The systems with amorphous (liquid) separation with upper and lower critical solution temperatures, crystalline phase separation and liquid-crystalline transitions are considered. The effects of molecular weight, chain flexibility, polymer concentration, polarity and sizes of solvent molecules, deformation rate, magnetic field intensity, interfacial energy on phase transitions in polymer systems under mechanical, magnetic and electric fields are analyzed. Phase diagrams and pulsating mechanism of phase separation of the gels of cross-linked polymers are described. The monograph "Phase Transitions of Polymer Systems Induced in External Fields" will be useful for scientificresearchers, postgraduate students and students specializing in the phase transitions in multicomponent systems.