As obligate cellular parasites, viruses do not have their own metabolism; therefore, they must infect cells for reproduction. The virus particles must be able to recognize specific receptor molecules on the cytoplasmic membrane of the host cell and to bind to them. This process is known as attachment. In enveloped viruses, this interaction is mediated by proteins that are embedded within the viral envelope. This is the case in influenza viruses as well as retroviruses and herpesviruses. Binding of viral envelope proteins to cellular surface structures is to some extent very specific: this is the case for the interaction between the surface protein gp120 of human immunodeficiency virus (HIV) and the CD4 receptor, a polypeptide that occurs almost exclusively in the cytoplasmic membrane of T-helper cells and macrophages (Sect. 10.1007/978-3-642-20718-1_18). In other cases, viral proteins bind to cellular structures that are found on many cell types. One example is the binding of haemagglutinin of influenza viruses (Sect. 10.1007/978-3-642-20718-1_16) to terminal N-acetylneuraminic acid residues of complex oligosaccharides, which are found as protein and lipid modifications on the membrane surface of various cells.