Spectroscopic ellipsometry is used to determine optical constants, layer thicknesses in multilayer stacks, and microstructure (voids, alloy fraction, or mixed phase composition), and is a well-developed technique for analysis of optical thin films. Ellipsometers now cover from 140 nm (≈ 9 eV) in the vacuum-ultraviolet to 200 microns (50cm-1) in the far infrared. Generalized anisotropy and depolarization are measurable using rotating compensator ellipsometers or controlled retarders for partial Mueller Matrix analysis. Rotating compensator ellipsometers allow accurate and rapid in situ diagnostics, including window birefringence calibration. Steady progress has been made in both ex situ and in situ ellipsometry hardware, software, and applications. These advances are reviewed and examples given.