© 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. A topological model of the formation of type II twins in NiTi martensite is presented. Disconnection dipoles are generated initially and expand on the rational k 1 = (011) plane, subsequently accumulating to form a tilt wall perpendicular to k 1 . Equilibrium occurs when the strain and rotational distortion fields of the constituent defects are equally partitioned between the adjacent crystals. The resultant interface is parallel to the irrational conjugate plane, (Formula presented.), consistent with the classical geometric theory of deformation twinning and previously published experimental observations. It is proposed that the formation of type II twins on k 2 occurs in this manner, rather than formation of type I twins on k 1 , because the disconnections have limited mobility on k 1 even though they are readily nucleated on this plane. We support this conjecture by showing that the Burgers vector of the defects has a small magnitude, implying easy nucleation, but their motion along k 1 is accompanied by complicated atomic shuffling.