In extrusion, mixing of solids and melts has always been problematic, leading to diverse models describing the melting process. It is found that for foods based on cereals, only a few are valid, due to the simultaneous presence of water and high viscous non-Newtonian material. Mixing trials are summarised for single and twin screw extruders, with particulate solids of different particle sizes like, maize grits, wheat flour, sucrose crystals and glucose syrup. Special mixing-heads for single screw extruders like, pineapple-heads, and slotted flight sections for counter-rotating twin screw extruders are investigated. The effects of screw geometry on mixing has been measured using co-rotating twin screw extruder modular elements like: single lead elements, mixing paddles and reversed pitch elements in a translucent model extruder by means of a study of flow phenomena and residence time distribution (RTD) measurements. Mixing effects are reported and their influences on viscous dissipation, residence time, curve spread and stagnancy are explained.