During the last years, the development of processes for 3D-printing has been driven forward. Besides optimizing the printing process and developing new types of processes by altering components of the printer for example, research is also being pursued to investigate new materials suitable for 3D printing, including extrusion-based processes. Most of the printing materials show non-Newtonian behaviour within certain limits. Understanding the rheology of these materials but also the impact on the whole printing process can be a key factor for the printing quality. The origin of this paper is the attempt to point out critical process parameters in the process development of extrusion-based processes using materials with non-Newtonian behaviour. Focus is set on four types of materials including polymers, slurries, foods and biomaterials. After a short summary of the state of the art, the critical process parameters are discussed. The present paper shows that parameters like a shear-thinning or thixotropic behaviour, the sort of the filler material as well as the temperature and yield stress are of great importance for all four discussed groups.