Advanced structural composites such as carbon and glass fibre-reinforced polymers have a limited ductility. This is due to intrinsic brittleness of the high performance fibres. The composite ductility can be enhanced by choosing fibers with a higher strain-to-failure, but most known ductile fibres (e.g. some natural and polymer fibres) have a low stiffness. The aim of this work was to investigate a new fibre type for application in composites annealed stainless steel fibres. The unique property of these fibres is that they combine a high stiffness (±193GPa) with a strain-to-failure which can be tailored up to 20%. The strain-to-failure is as high as that of a silk fibre and up to 10 times higher than that of a carbon fibre.An extensive experimental program, supported by modelling investigations, was performed to understand the mechanical behaviour of polymer composites made of these fibres. The influence of the matrix ductility, fibre architecture and interphase properties on the damage development and composite properties was investigated. Additionally steel fibre hybrids in combination with carbon fibres, glass fibres or drawn polypropylene tapes were studied.The ductile steel fibres delivered composites with a high stiffness and a high strain-to-failure. The strain-to-failure can be up to 22%, which is much higher than a typical carbon and glass fibre composite (10 and 5 times, respectively).Steel fibre composites with ductile matrices showed the highest strain-to-failure and a distinctly different failure behaviour, compared to a steel fibre composite with a brittle matrix. In both cases the strain-to-failure could be further improved using a silane treatment which increased the adhesion strength.By replacing low amounts of steel fibres by carbon or glass fibres, the composite strength increased, but with a significantly lower strain-to-failure. However, the sudden failure which is typical for carbon and glass fibre composites, was not present.Adding steel fibres to self-reinforced polypropylene increased its stiffness and, also its specific stiffness despite the high density of steel. In contrast to hybridisation with a brittle fibre, no loss in strain-to-failure in a tensile test was measured.