The lecture deals with the design of high entropy alloys where metastable phase states are not coincidentally inherited from processing, but rather are engineered. Phase stability can be tuned by chemical composition (thermodynamics, e.g. partitioning), tempering (kinetics, e.g. nucleation), and microstructure (confinement, e.g. size effects). When exposed to loads metastable phases can trigger athermal transformation effects such as TRIP and TWIP. The concept works at the bulk scale and also at a spatially confined microstructure scale, such as at lattice defects. In the latter case, local stability tuning works primarily through segregation engineering, i.e. through targeted elemental partitioning to dislocation cores, stacking faults, interfaces, and precipitates with the aim to render only these confined regions metastable. Depending on stability, spatial confinement, misfit, and dispersion, both bulk and local load-driven athermal transformations can equip high entropy alloys with substantial gain in strength, ductility, and damage tolerance.