This publication summarises the current state of knowledge and technology on the possibilities and limitations of using mineral and synthetic fillers in the field of 3D printing of thermoplastics. FDM technology can be perceived as a miniaturised variation of conventional extrusion processing (a microextrusion process). However, scaling the process down has an undoubtful drawback of significantly reducing the extrudate diameter (often by a factor of ≈20–30). Therefore, the results produced under conventional extrusion processing cannot be simply translated to processes run with the application of FDM technology. With that in mind, discussing the latest findings in composite materials preparation and application in FDM 3D printing was necessary.