This monograph discusses a new class of fiber reinforced composites called as microfibrillar composites(MFCs)prepared from two fibre forming thermoplastic polymers. MFC technology provides an excellent method to make use of commingled plastics in which both low melting temperature general plastics and high melting temperature engineering plastics co-exist. In this work, the properties of MFCs prepared from polypropylene (PP)/ polyethylene terephthalate (PET) and low density polyethylene (LDPE)/PET blends were analyzed. In the case of PP/PET MFCs the effect of draw ratio on the morphology, static and dynamic mechanical, thermal, rheological properties were studied. The MFCs prepared at the optimum draw ratio exhibited significant increase in the tensile and flexural moduli in comparison with the conventional blends. In LDPE/PET system, the effect of blend ratio and the effect of compatibilizer content on the properties of normal blends,microfibrillar blends and composites were evaluated. The static and dynamic mechanical, solvent sorption and thermal degradation properties of the microfibrillar blends/composites were found to be superior to conventional LDPE/PET blends.