Publisher Summary The chapter discusses the viruses of invertebrates. The great majority of the viroses described among the invertebrates occur in the Insecta. In the Lepidoptera, newly found nuclear polyhedroses, intestinal cytoplasmic polyhedroses, and granuloses have been added to the list of inclusion body viroses. Most of the viral diseases described in the recent years in invertebrates belong to types of viroses already known. On the other hand, cases arise of types, which, because of their pathogenicity or viral characteristics, constitute groups that are not yet known among the invertebrates or even in the field of general virology. These new types are briefly mentioned. The chapter discusses insect virus inclusion bodies divided into three types: nuclear polyhedra, cytoplasmic polyhedra , and granules, a classification that takes into consideration the shape, the intracellular and tissue distribution, and the nature of the viruses they contain. Purification techniques of viruses are presented. The purification of virus suspensions has generally been achieved as the result of a trial-and-error approach, and with the exception of the viruses from polyhedroses and granuloses the purification of which has been codified; no scheme of separation useful for all viruses can be put forward. Nevertheless, repeated high- and low-speed centrifugation cycles constitute the most widely used method, purification usually being completed by the means of density-gradient centrifugation. The chapter discusses on the physicochemical properties and chemical analyses of insect viruses.