X-ray reflectometry (XR) is a powerful tool for the evaluation of the nanostructure of the adsorbed polymer layers at the air-water interface. We newly established the Air/Water Interface X-ray Reflectometer for laboratory use and have performed in-situ XR measurements for various kinds of spread monolayers. In this review, we report the measurements for electrolytic amphiphilic diblock copolymer, “command surface type” photochromic polymer/liquid crystal complex, and lipid/protein complex on a water surface. From XR data, the layer thickness and density profile were determined precisely. The XR results will not only reveal the structure of the adsorbed layers at the surface and interface, but also provide the new knowledge about the relationship between the structure, assembling mechanism, and the functionality. In addition, we have examined the application to detect the capillary wave on the water surface using the XR technique.