The co-rotating intermeshing twin screw extruders are used widely in processing and compounding of polymers. The development of the first conical tightly intermeshing co-rotating twin screw extruder for processing polymer by the company Maschinen und Anlagenbau Schulz GmbH (M-A-S) in 2007 will change the general classification of the twin screw extruders, which till now excludes the conical co-rotating system. The new conical technology has several advantages over the parallel co-rotating twin screws. The main advantage is the large intake volume in the feeding zone, due to the conicity of the extruder, which facilitates the usage of the conical extruders for processing or recycling or compounding materials with very low bulk density, such as film flakes, recycled bottles or wood plastic composites etc. For an optimal utilization of the parallel or conical co-rotating extruders an in depth knowledge on the processing characteristics of the plasticizing screws are very important. Moreover the quality of the end product is highly influenced by the screw geometry and the configuration. The configuration of the screw is generally based on experiences or applying the “trail and error” method, which is time and cost consuming. The easiest and faster way is to use mathematical process models to predict the processing behavior of the configured screw without carrying out any practical trails. Such process models are available for parallel co-rotating screws. Since the conical co-rotating twin screw extruders are new to the extrusion sector, until now no studies have been carried out on these conical screws for processing polymer materials. The main focus of this thesis was to develop mathematical process models for the conical co-rotating twin screws, based on the currently available models for parallel screws and to verify the developed models with practical results. Different screw elements and different processing zones of the parallel and conical twin screw extruders were theoretically and experimentally analyzed. With the developed mathematical models for conical screws in this work, a software “NCT” was developed for computing the processing behavior of conical co-rotating twin screws with user defined screw configuration and processing parameters. A similar simulating software “2SX” for parallel co-rotating twin screw extruder, especially for the compounders available at the Chair of Polymer Processing, Montanuniversitaet Leoben was developed as a part of this work. With this software, screw configuration or individual screw elements can be simulated and optimized prior to experimental trails.