Both deformation twinning and martensitic transformation may be regarded as modes of deformation, and models which relate the nucleation and growth of twins and martensite plates to dislocations and stacking faults are reviewed. Recent experimental and theoretical work indicates that b. c. c. twins mays originate from screw dislocations as three- or four-layer faults, and that the stress-assisted nucleation of successive loops of twinning or transformation dislocation may be much easier than indicated by elastic theory. The propagation of slip across coherent interfaces sometimes involves the combination of groups of individual lattice dislocations and the new slip system may be non-conventional ; similar considerations apply to twin-twin intersections. The relation of the interface structure of semi-coherent martensite plates to single crystal, faulted single crystal and twinned products is discussed, and recent work on stress-assisted and straininduced martensite is described.