Ellipsometry is an optical method used for characterizing materials and thin films. The principle is based on the polarization change at a sample due to the reflection or transmission at boundaries. By the measurement of the amplitude ratio psi and the phase difference delta, the complex refractive index can be obtained. Ellipsometers can be used in various industries, e.g., semiconductor, chemistry, and display industry. The typical applications are quality control of film growth and defect inspection. In the configuration of return-path ellipsometry (RPE), the light beam is reflected twice from the sample. Thus, RPE has a higher sensitivity to the optical properties of samples. Some configurations of RPE have high tilt tolerance which is an important requirement for inline measurement. This report gives an introduction to the principle of ellipsometry and an overview of four different types of RPE.