Thermodynamic aspects and mechanisms of t h e deformation of thermoelastoplastic block copolymers of t h e styrene-diene, segmented polyurethane and polyest e r types a r e discussed. Deformation calorimetry was used t o study energy and entropy e f f e c t s r e s u l t i n g from t h e deformation and small angle X-ray s c a t t e r i n g method was used t o obtain information concerning s t r u c t u r a l changes during deformation. The larges t r a i n t e n s i l e deformation of thermoelastoplastics which possess domain s t r u c t u r e l e a d s t o a " p l a s t i c --rubber" t r a n s i t i o n ( s t r e s s softening e f f e c t ) , which under appropriate time and temperature conditions i s r e v e r s i b l e . A comparative thermodynamic a n a l y s i s of t h e s t r e s s softening i n block copolymers and f i l l e d rubbers as c a r r i e d ' o u t and r e s u l t s d e a l i n g with recovering k i n e t i c s of i n i t i a l domain morpholog i n s t r e s s -s o f t e n e d block copolymers a r e discussez. P a r t ic u l a r a t t e n t i o n i s paid t o t h e s t r u c t u r a l changes and thermodynamics of deformation of " s i n g l e c r y s t a l " morphology,which can a r i s e i n diene-styrene block copolymers under appropriate thermo-mechanical treatment. The r o l e of the chemical nature of s o f t and hard blocks a s well a s the morphology and s i z e of hard blocks i n thermodynamics and mechanisms of the deformation i s analyzed. The problem of t h e l i m i t e d e x t e n s i b i l i t y of s o f t block polymer chains a t high deformations i s a l s o discussed.