X-ray crystallographic analyses, together with nuclear magnetic resonance, have revealed three-dimensional structures of many important viral proteins, thereby allowing us to better understand the interactions between viral and host cell molecules. In this review, we summarize the recently determined crystal structure of the measles virus (MV) attachment protein hemagglutinin. Based on this structural information, we also discuss how the MV hemagglutinin interacts with various cellular receptors and why MV vaccines have been effective for many years without inducing escape mutant viruses. Other topics discussed are a putative MV receptor present on polarized epithelial cells and the protein expression system using a cultured human cell line 293SGnTI(-), which is suitable for X-ray crystallographic analyses.