Interpenetrating network polymers (IPNs) are chemically homogeneous (on a sciently large scale binary systems) that can be conveniently prepared by swelling first the loosely crosslinked component in a suitably chosen monomer and subsequently crosslinking the latter. The properties of blends and IPNs are dependent on the two-phase nature, phase continuity, domain size and molecular mixing at phase boundaries. A study of morphology, glass transition temperature, modulus, etc., explains the properties of any two-component system. Natural rubber (NR) is a typical elastomer having good resilience, elongation and elasticity, ease of storage, milling and the possibility of using different vulcanization processes. Semi-IPNs based on NR/polystyrene (PS), where only the NR phase is crosslinked, offer a binary polymer system network having the elastomeric properties of NR imparted to the hard brittle properties of PS. Physical blends of NR and 1,2-polybutadiene (1,2-PBD), a thermoplastic rubber, combine the good physical and superior ageing properties of 1,2-PBD with the unique elastomeric/hysteresis characteristics of blends based on NR and ethylene-vinyl acetate copolymer (EVA) (physical blends and statically vulcanized) combine the good elastomeric and mechanical properties of NR with the excellent ageing and flex crack resistance of EVA.