In general, polymer blends are most often developed in order to raise toughness of conventional polymers and to overcome processing difficulties of certain polymers. These enhancements however, can be obtained providing good compatibility between blend components by purposeful selection of the blend components, compatibilization or modification in the case of necessity and choosing the most appropriate processing regimes. In order to satisfy the demands of new technologies there is growing incentive of using new polymer grades with more regular structure and better predictable and reproductable macromolecular characteristics for developing of polymer-polymer composites. Thus new polypropylene (PP) grades with optimized crystallization behaviour and metallocene ethylene copolymers with stable easy controllable structure and narrow molecular weight distribution and high co-monomer arrangement regularity are perspective candidates for building of new polymer-polymer composites. Besides it, due to structural affinity of PP with ethylene polymers no major technical problems for the compatibility development between the components are expected. Development of polymer blends with stable commercially attractive exploitation properties, however, is highly dependant also on the proper choice of technological processing conditions, considering thermo-oxidative stability, rheological characteristics and crystallization specifics of the blend components. If these tasks are successfully solved developed polyolefin composites potentially can be widely used in different branches of national economy, including building, automotive, consumer a.o. sectors.