The lattice relaxation of strained Si1-xGex layers on Si (001) substrates has been examined. Single layers having a nominal composition of x = 0.14 were grown by Molecular Beam Epitaxy to thicknesses of 0.5, 1.0 and 1.5 μm, all of which are greater than the critical thickness where misfit-dislocation generation commences. Double-crystal and white-radiation topographic methods were used to reveal the misfit dislocation structure and distribution. The misfit dislocations were shown to extend from heterogeneous nucleation sites along the four available <110> directions in the plane of the interface. A symmetric distribution of dislocations between the orthogonal < 110> directions was observed. Secondary branching of the misfit dislocations was also observed which accelerates the relaxation process.