The recent use of fiber optic waveguides for biochemical analyses is based mainly on light intensity changes. Waveguide evanescent field ellipsometry (WEFE) is proposed for biosensor applications. The WEFE is based on the principle of ellipsometry. A laser beam coupled into the waveguide propagates in the waveguide with two fundamental modes with transverse electric fields parallel and perpendicular to the sensing surface. The evanescent waves interact with the adsorbed or conjugated layer of biomolecules, producing changes in the relative phase and amplitude upon interaction. The ellipsometric parameters change with biomolecule adsorption or conjugation on the sensing surface. From the ellipsometric parameter changes, we can estimate the surface concentration and thus the concentration of the biomolecules in solution. The principles of WEFE including light propagation in the waveguide, ellipsometry parameter analysis, the WEFE arrangement, materials and methods, and preliminary experimental results are presented.