Although single clay particles can seldom be seen with an optical microscope the preferred orientation of an aggregate of clay particles may be investigated by studying in thin section the birefringence of the aggregate. The techniques of preparing thin sections of clay with a minimum of disturbance of the structure and for measuring the birefringence under crossed nicols are described. It is shown that the birefringence of the aggregate arises solely from the birefringence of the constituent particles at least for the porosities of the clay used in this study. If a model distribution function for the spatial orientation of the particles be adopted the birefringent behaviour of the aggregate may be predicted and birefringence observations may be used to interpret the degree of orientation of an aggregate in terms of a model parameter. Two- and three-dimensional distribution functions are considered and the two-dimensional theory is judged to be preferable.