This chapter focuses on Iflaviridae family whose sole member genus is Iflavirus. The virions are roughly spherical and exhibit icosahedral symmetry with a diameter of 26–30 nm and have no envelope and no distinctive surface structures. The virions contain one molecule of linear, positive sense, and single stranded RNA, and the genome has a size ranging from 8800 to 10,100 nt and contains a single large ORF encoding a polyprotein of 2858–3085 amino acids. All proteins arise by proteolytic cleavage of a single polyprotein and mature virions contain three major structural proteins (VP1, VP2, and VP3) generally between 28 and 44 kDa. A fourth smaller capsid protein (VP4) of around 4–12 kDa has been reported in some species and is located in the second position of the capsid precursor coding region. The genome is monopartite and contains a large ORF with genes encoding capsid proteins at the 5´ end and genes encoding the nonstructural proteins at the 3´ end of the genome. The nonstructural proteins include a RNA helicase, a 3C-like cysteine protease, and a RNA-dependent RNA polymerase. The viral RNA is infectious and serves as both genomic and viral mRNA. The replication of viruses occurs in the cytoplasm of infected cells but the mechanism of viral genomic RNA entry into the host cell cytoplasm is unknown. The genomic RNA is translated into a polyprotein, which is autocatalytically cleaved into structural and nonstructural component peptides. All member viruses have been isolated from arthropod species and appear to have restricted host ranges. Since only one genus is currently recognized, the family description corresponds to the genus description.