We demonstrate that spectral ellipsometry is a suitable optical tool to investigate biological samples such as liver tissue, human nails, and human skin. We have performed ex-vivo as well as in-vivo measurements to obtain via the measured optical parameters psi and Delta information's such as layer thicknesses or hydration conditions. As an example for in-vivo studies, a tape stripping study of human skin is shown. Furthermore, we present a hydration study of nails over 48 h following a hydration schedule. The dehydration behavior of the nail is compared with the dehydration behavior of liver tissue. By investigating the change of (n) over tilde upon hydration and dehydration, we reveal three different time domains, corresponding to 'free' and 'bound' water and their respective rate changes. (C) 2004 Elsevier B.V. All rights reserved.