This is the fifth article in the series of X-ray thin-film measurement techniques. The second, third and forth articles of this series, previously published in the Rigaku Journal, describe out-of-plane, high-resolution and in-plane XRD measurements to obtain crystallographic information on crystal size, lattice strain and orientation relationship of a thin-film material. These measurements have been based on the premise of a crystalline thin film. On the other hand, the X-ray reflectivity (XRR) measurement is not a technique to evaluate diffraction phenomenon. The XRR measurement technique described in this article is used to analyze X-ray reflection intensity curves from grazing incident X-ray beam to determine thin-film parameters including thickness, density, and surface or interface roughness. This article will provide an overview of the principles of X-ray reflectivity, measurement procedures, and analysis methods. It also discusses the procedural flow from measurement to analysis, as well as precautions.