Indium tin oxide (ITO) is a transparent conducting oxide in wide use today, ITO can be difficult to work with since this material displays a complicated (graded) microstructure, and the optical properties of ITO can vary widely with deposition conditions and post-deposition processing. For this reason it is common to characterize ITO films via optical measurements. However, accurate results are difficult to obtain due to the graded microstructure of the film introducing variations in the refractive index throughout the film thickness. Thus the typical ITO film does not have a single, well-defined set of optical constants due to grading in the microstructure. Several optical models for ITO will be presented which include the graded microstructure of the material and work reasonably well in fitting spectroscopic ellipsometry data for ITO film thickness, index grading, and optical constants. Since the film thickness, optical constants, and microstructure grading are all intermixed in the experimental data the issue of determining a unique best-fit optical model for ITO will also be discussed.