In the past 10-15 years the need for the development of specifically engineered .structures of polymers has resulted in the proliferation of products manufactured using various coating and multilayer extrusion technologies. Initializing these technologies, great advantages can be achieved by combining polymers of widely dissimilar solid-state structures and properties into unique multilayer blends and composites. However, the manufacture of these products is not without problems. Many processes are currently hindered due to the occurrence of irregular interfaces and undesirable layer distribution. These irregularities are due to the different rheological properties of the material used and can significantly deteriorate the mechanical or optical properties of the product. Hence, to establish guidelines for the overall processing of multilayer coating and extrusion processes, a better understanding of interfacial instability and deformation of multilayer flow of polymeric fluid is required. In this research, an experimental apparatus has been developed for observing interfacial stability and deformation of multilayer pressure driven channel flows. This apparatus has been used to directly observe the deformation of the interface and growth of interfacial waves in order to enhance our understanding of the effect of viscoelastic forces on stability and deformation of multilayer viscoelastic flow. Due to synergetic effects of combining individual polymers, multilayer films and conjugated fibers have been progressively gaining interest in the past decade. In the manufacture of multilayer products, it is a well-known fact that the multilayer extrusion process is more economical than the conventional laminating process. These products are generally superior to their single component counterparts because of the combining materials with different properties into a single structure. This effect is readily observed in the food packing industry where packing films contain multiple layers, each of which contribute a specific barrier, mechanical or optical property to the resultant film. Interfacial instabilities that manifest themselves in form of traveling waves at the interface are the limiting factor in production of multilayer plastic structures. These interfacial waves result in a significant deterioration of the final products properties of interest (i.e. barrier, mechanical, optical, etc.), hence to better design and control these processes a comprehensive understanding of the interfacial instability phenomenon is required. This would facilitate the selection of materials with suitable rheological properties, proper design of co extrusion die and proper selection of layer ratio within the structure.