During the past two decades, virus taxonomy has advanced to the point where most viruses can be classified as belonging to families, genera, or groups of related viruses. Virus classification is primarily based on chem­ ical and physical similarities, such as the size and shape of the virion, the nature of the genomic nucleic acid, the number and function of com­ ponent proteins, the presence of lipids and of additional structural fea­ tures, such as envelopes, and serological interrelationships. The families, genera, or groups of viruses that have been defined on the basis of such criteria by the International Committee on Taxonomy of Viruses (ICTV) will be described in some detail in this catalogue and illustrated by elec­ tron micrographs. In my present attempt to list most if not all well es­ tablished and studied viruses in alphabetical order, I have largely confined myself to identifying them only in such taxonomic terms, generally without quoting specific data reported for individual viruses. If the latter data do not at times agree closely with those given for the taxon or group, it is difficult to decide to what extent this is attributable to misclassi­ fication due to insufficient data and errors in the analytical procedures and descriptions, or to what extent this is an expression of Nature's free­ dom of choice and abhorrence of restrictive classifications.