Complex concentrated alloys (CCAs) extend the compositional paradigm shift of high entropy alloys (HEAs) to new microstructural opportunities. CCAs provide opportunities for tunable performance by manipulating deformation mechanisms. Fe–Mn–Co–Cr–Si alloys exhibit potential for a combination of phase transformation and twinning. These alloys give greater flexibility for tailoring transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP), which have guided design of next-generation steel alloys over the last 20 years to a new level. For TRIP CCAs, the ductility can be extended to as high as 50% while maintaining a strength exceeding 1 GPa. The Fe–Mn–Co–Cr–Si alloys show extensive γ (f.c.c.) to ε (h.c.p.) phase transformation followed by additional twinning in the epsilon phase. Combination of ultrafine grained microstructure and crack tip transformation leads to enhanced fatigue limit in TRIP CCAs. Design of non-equiatomic CCAs provides a vast, and vastly unexplored compositional space for developing new alloys with tunable properties. © 2020 Elsevier B.V.