In thin film research, in order to understand comprehensively the physical properties of the samples, it is essential to have knowledge about the crystallographic, compositional as well as the structural properties of the samples. X-ray reflectivity (XRR) is an ex situ non-destructive technique that allows one to investigate thin film characteristics such as the layer thickness, t, interface roughness, σ, and the layer density, ρ. This is an absolute technique that does not require calibration samples. X-ray reflectivity is sensitive to changes of the average electron density across an interface. Because of this, the technique can be performed on non-crystalline and crystalline samples. In this webinar, a general introduction to X-ray reflectivity and general conditions to perform XRR measurements on a laboratory diffractometer are presented.