Present knowledge of cell surface receptors for animal viruses is reviewed. The methods used for enumeration and identification of receptors are critically examined with respect to particular advantages and disadvantages. Specific controls and alternative interpretations are suggested in connection with the reactions of lectins which block viral attachment to cells. Currently available information for each group of animal viruses is summarized in order to define the extent to which the corresponding receptors have been identified. It is concluded that the full range of virus-receptor interactions has not yet been explored even for those viruses of which there is the most detailed knowledge. For some groups, moreover, the receptor is totally uncharacterized. Six areas in which future investigative effort might be productive are identified, including the isolation of membrane components and the immunochemical definition of viral receptors.