Two types of interfaces can be distinguished in TiAI polycrystals: special rotational interfaces on the {111} planes for which the misorientation angles about the normal to the interface plane are multiples of 60 degrees; and grain boundaries in common sense with arbitrary orientations of the boundary plane and rotation axis. The former are the boundaries in the lamellar structure parallel to one of the close packed atomic planes and the latter separate differently oriented colonies of such lamellae. The effect of both these interfaces on the plastic deformation of TiAI intermetallics is discussed. In particular, the lamellar interfaces as strong obstacles for moving dislocations induce a specific type of combined plastic deformation when the glide of ordinary dislocations operates simultaneously with twinning. On the other hand, the grain boundary can affect the dislocation processes at its vicinity by the arising compatibility stresses. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.