A lecture given by Ivan Gutierrez-Urrutia, at the Adventures in the Physical Metallurgy of Steels (APMS) conference held in Cambridge University. Low-density steels are described. We present an overview of the ongoing activities on low-density steels at MPIE. We have developed two alloy concepts of high strength and ductile lightweight steels, namely, Simplex and Kappa steels. Simplex steels are austenitic Fe-Mn-Al-C steels with high Mn (~30 wt.%) and C (~1.2 wt.%) contents. These alloys exhibit exhibits a superior combination of strength and ductility (ultimate tensile strength of 0.9 GPa and elongation to failure of 80%) due to multiple-stage strain hardening. The development of an evolving hierarchical deformation structure consisting of dislocation configurations and twin substructures results in an outstanding permanent strain hardening. The role of the alloying elements on the underlying strain hardening mechanisms will be presented. Kappa steels are austenitic Fe-Mn-Al-C steels with high Mn (~30 wt.%), Al (~8.0 wt%) and C (~1.2 wt.%) contents. These light-weight alloys contain high volume fractions of L12-type ordered nanoprecipitates, so-called kappa carbides, conferring not only high strength (ultimate tensile strength of 1.2 GPa) but good ductility as well (elongation to failure of 40%). Multi-scale characterisation of nano-sized kappa carbides by TEM and 3D-APT provides new insights on kappa carbide-dislocation interactions.