The total polarization fields of pseudomorphic In xGa 1-xN/GaN and Al xGa 1-xN/GaN heterostructures with 0≤x≤0.4 have been calculated as a function of the crystal orientation. Especial attention is placed on the direction and magnitude of in-plane piezoelectric polarization, which is not negligible for the nonpolar and semi-polar growth. For an arbitrary crystal orientation, the piezoelectric polarization prevails in the InGaN/GaN system while the spontaneous polarization prevails in the AlGaN/GaN system. The in-plane potential due to polarization fields in non-polar epilayers is found to depend on the degree of planarity of the heterojunctions, and on the respective lateral dimensions. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.