By a detailed consideration of the geometrical consequences of shear displacements in Ll2 γ' and the requirement of compatibility with the fcc γ matrix, it has been possible to define the shear modes potentially operative in γ' precipitation hardened nickel-base alloys. Most of these geometrically possible deformation modes have, in fact, been observed. The predominant mechanisms are planar glide on { 111 } of paired a/2 < 110 > dislocations at low temperature and viscous cutting of γ' by paired a/2 < 110 > in climb configurations at high temperature. The a/3 < 112 > stacking fault modes of shear are of secondary importance, except in primary creep at intermediate temperatures where the motion of superlattice intrinsic/extrinsic stacking fault pairs is rate controlling