In this contribution, we review the analysis of ellipsometric spectra of general anisotropic materials from an experimental point of view. The basic framework of data analysis is the numerical solution of the exact ellipsometric equations by using the 4 x 4 transfer matrix algorithm. The flexibility of our approach allows undertaking the study of the dielectric tensor in general systems. We illustrate the practical use of our method with different examples. In particular we discuss the study of systems of reduced symmetry where principal axes of the dielectric tensor are a priori unknown, as monoclinic alpha-PTCDA and anthracene. (C) 2003 Elsevier B.V. All rights reserved.