Co-rotating twin-screw extruders are widely used compounding machines. They are mainly configured based on extensive experience and iterative approaches to optimise output and composite quality. The visualisation technology developed in the EU-project PEPTFlow allows visualisation of composite flow in twin-screw extruders under realistic processing conditions by tracking radioactive tracer particles in the polymer melt, using a specially developed camera system. This new approach allows polymer flow to be studied in different screw elements and screw configurations under realistic compounding conditions at normal temperatures and melt pressures. The paper presents the latest developments in the camera systems as well as the different ways to use and interpret the results. Detailed analysis of residence times and residence time distributions for standard compound screw elements, like kneading discs, conveying elements and reverse elements are presented. In addition for a better understanding of the flow field inside twin-screw extruders, numerical particle tracking is done. The Stokes equation, using XFEM method, are solved and the numerical RTD's (residence time distribution) are compared for various screw designs.