Natural fibre reinforced thermoplastics are usually processed in twin screw extruders. The present work studies the single screw extrusion process for different systems of natural fibre reinforced thermoplastics. For this study different polymer-fibre systems have been characterised in terms of the material properties that are relevant to the processing behaviour of these materials. The extrusion process has been investigated using an open-barrel single screw extruder. This equipment can be stopped at any point in operation, cooled down and opened in order to study the material transport processes that take place inside the extruder. The polymer matrices used in this study have been mainly polyethylene and polypropylene (with MFIs varying from 0.3 to 4). Fibres including jute, sisal, and cabuya have been used as discrete reinforcements of a length of 5 to 10 mm. The experiments presented here study the transport processes during single screw extrusion with special emphasis on the bubble formation mechanisms. The bubble formation process has been studied in terms of the variation in the initial fibre dispersion at the entrance of the feeding zone of the extruder A relationship between solids conveying, plastication and bubble formation has been proposed considering the different typical cases occurring in the single screw extrusion of NFRTPs. The use of coupling and dispersion agents has been studied in terms of its relationship to bubble formation and to the final quality of the extrudates produced. Finally, morphological characterisation techniques have been adapted to understand the structure-property relationships in these materials during the extrusion process.