Extrusion-based additive manufacturing (EB-AM), which is also known as fused filament fabrication or 3D-printing, is an emerging processing technique that is characterised by the selective deposition of thermoplastic filaments in a layer-by-layer manner based on digital part models. In recent years, it has attracted considerable attention both from industry and research, as this technique offers manifold benefits over conventional manufacturing technologies in terms of flexible production, freedom of design, independency of cost-intensive moulds, and a reduced time to market. However, to meet the challenges of increasingly complex industrial applications, certain shortcomings of EB-AM still need to be overcome. A case in point are the generally inferior mechanical properties of 3D-printed parts compared to those of conventionally processed parts. Moreover, despite rigorous research activities, certain semi-crystalline thermoplastics such as polypropylene (PP), which offer attractive properties, are still not established as reliable, commercially available filament materials. The present PhD thesis attempts to close these gaps by exploring various effective strategies to enhance the mechanical properties of 3D-printed parts in general, and to overcome the main limitation of PP processed by EB-AM, namely its susceptibility to shrinkage and warpage. In a first step, the adhesion of the first deposited layer on the build platform was optimised by means of in-situ shear-off measurements and surface analyses, as this was found to be a prerequisite for controllable warpage for any filament material. In the case of PP, a substantial warpage reduction of 3D-printed parts was achieved by incorporating up to 30 vol.-% of low aspect ratio fillers. In order to maintain the novel materials’ processability, their mechanical, thermal, and rheological properties were optimised by improving their morphology through the addition of compatibilisers, filler coatings, and amorphous polyolefins. A further improvement of the mechanical and warpage properties was obtained by exchanging the low aspect ratio fillers with thermally conductive carbon fibres and by increasing the printing chamber temperature to homogenise the temperature distribution within the printed parts. The most promising PP-composite developed in this work resulted in negligible warpage and greatly increased tensile and flexural strengths compared to neat PP. To explore further ways of improving the mechanical properties of 3D-printed components, the weld strengths of both additively manufactured PP-composites and poly(lactic acid) were optimised through statistical parametric investigations, and characterised by means of adapted conventional and fracture mechanical testing techniques. When the recommended process parameters were met, the strength as well as the fracture toughness under both static and dynamic loading conditions were found to be comparable to those of parts processed by compression moulding.