A unified theory for light scattering by biological cells is presented. It is shown that Mie scattering from the bare cell and the nucleus dominates cell light scattering in the forward directions. The random fluctuation of the background refractive index within the cell, behaving as a fractal random continuous medium, dominates light scattering by cells in other angles. The theory is validated by experimental angular light scattering spectra of epithelial cells for scattering angles from 1.25 to 173.8 degrees and in the spectral range from 400nm to 700nm.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.