Multiple strengthening and toughening mechanisms can be combined in the multicomponent high-entropy materials (HEMs) to achieve excellent mechanical properties within their practically infinite compositional space. For instance, apart from the intrinsic massive solid solution strengthening effect, a joint activation of twinning- and transformation-induced plasticity effects can be realized in a series of high-entropy alloys (HEAs) by rendering them metastable. In this talk, we will introduce some of our recent ideas associated with the metastability engineering for strengthening and toughening multicomponent HEMs. For examples, an unexpected sluggish martensitic transformation can exhibit in strong and super-ductile HEAs of ultralow stacking fault energy (~7 mJ/m2), where chemical short-range orders and lattice distortions play roles in the deformation. A novel “high-stress twinning” behaviour can be triggered in ultrastrong HEAs of very high stacking fault energy (~80 mJ/m2), which in turn promote the good ductility. Further, a concept of strengthening and toughening high-entropy ceramics (HECs) by incorporating with dispersed metastable ceramic phases can be realized with the metastability engineering strategy. The associated mechanisms behind these ideas will be clarified and briefly discussed.