The ultimate goal of structural analysis of any virus is not only to delineate its molecular structure but also to understand the structure in terms of its functional parts. In recent years, three-dimensional structures of several animal viruses have been determined using X-ray crystallography (Hogle et al. 1985; Rossmann et al. 1985; Acharya et al. 1989; Harrison 1990) and electron microscopy (Crowther and Klug 1975; Vogel et al. 1986; Fuller 1987; Prasad et al. 1988; Baker et al. 1988; Schrag et al. 1989) . These structures have provided greater insight not only into the architectural principles of viruses but also into the molecular mechanisms such as entry of viruses into cells, virus disassembly, assembly, and neutralization (Harison 1990; Rossmann and Rueckert 1988; Rossmann 1989; Fuller 1987; Prasad et al. 1990). In some cases, structural analysis of the viral components has also been very useful. Structural studies of glycoproteins of influenza virus have provided a good deal of information relating to receptor binding, membrane fusion, and antigen-antibody interactions (Wiley and Skehel 1990; Colman et al. 1987). It is to be expected that such structural studies on viruses and viral components will prove useful in the rational design of antiviral drugs and vaccine development.
Rotavirus is a large spherical virus which has been identified as a major cause of infantile gastroenteritis (DeZoysa and Feachem 1985; Kapikian and Channock 1990). Ever since its discovery in 1973 (Bishop et al. 19873), this virus has been a subject of several structural studies using conventional electron microscopy (Martin et al. 1975; Espaarza and Gil 1978; Kogasaka et al. 1979; Roseto et al. 1979). Recently, the three-dimensional structure of rotavirus has been determined using electron cryomicroscopy and computer image processing techniques (Prasad et al. 1988, 1990). In the following sections we briefly discuss these techniques and follow with a detailed description of the three-dimensional structure and its implication on the functional aspects of the virus.