The paper deals with some fundamental issues essential for constitutive modelling of plastic behaviour of metals. Geometric and kinematic aspects of intragranular as well as intergranular plastic deformation of polycrystals are discussed. Homogeneous grain strains are composed into the resulting behaviour of representative volume element (RVE). A homogenization of total, plastic and elastic strains has been done. Constitutive equations by a self consistent method have been discussed. A simplest case of higher gradient theory is discussed. Elastic strain is covered by the efiective field homogenization method inside a RVE. It is underlined that plastic stretching and plastic spin are not independent.