Introduction. Study of the initial stages of the interaction between animal viruses and their host cells has been overshadowed by interest in virus-directed macromolecular syntheses. However, increasing understanding of these processes has shown the difficulty of inhibiting them in vivo. The result is a resurgence of interest in the initial stages of infection, defined here as those virus-cell interactions which precede virus-directed synthetic processes. Despite incomplete data, hypotheses are taking shape and an appraisal of the situation seemed timely. This review will avoid the historical approach and focus on areas of particular interest or controversy. Early work, particularly morphological studies, has been thoroughly covered by others and will only be mentioned selectively (for details, consult Dales, 1965, 1973; Burrows, 1972; Lonberg-Holm & Philipson, 1974, 1980; Bächi et al., 1977; Howe et al., 1980; Kohn, 1979).