The family Orthomyxoviridae contains four genera: Influenzavirus A, Influenzavirus B, Influenzavirus C, and Thogotovirus. Influenza A virus is the virus that infects humans and a variety of avian species, horses, and pigs. Influenza A virus caused the major pandemics of respiratory illness during the twentieth century. Influenza B virus strains appear to infect naturally only humans and caused epidemics every few years. Influenza C virus causes more limited outbreaks in humans and also infects pigs. The natural transmission of influenza A, B, and C viruses is by aerosol for humans and nonaquatic hosts whereas transmission is waterborne for aquatic species. Thogoto viruses are transmitted by ticks and replicate in both ticks and in mammalian species and are not discussed further here. Influenza viruses are enveloped viruses, often spherical (80 to 120 nm in diameter) but also filamentous (several micrometers in length). The lipid envelope is derived from the plasma membrane of the cell in which the virus replicates and is acquired by a budding process from the cell plasma membrane as one of the last steps of virus assembly. Inserted into the virion envelope are viral glycoprotein spikes (10 to 14 nm in length). For influenza A and B viruses, the surface spike glycoproteins are hemagglutinin (HA) and neuraminidase (NA). The envelope also contains small amounts of a proton-selective ion channel (M2 for influenza A virus and BM2 for influenza B virus). Influenza C virus contains only one spike glycoprotein, hemagglutinin-esterase-fusion (HEF). Influenza A, B, and C viruses contain a matrix (M1) protein and it is now virtually dogma that the matrix protein underlies the lipid envelope and provides rigidity to the membrane. In addition, it is widely believed that the M1 protein interacts with the cytoplasmic tails of the HA, NA, and M2 (or BM2) proteins and also interacts with the ribonucleoprotein (RNP) structures, thereby organizing the process of virus assembly. The viruses contain a segmented single-stranded RNA genome (influenza A and B viruses, eight RNA segments; and influenza C viruses, seven RNA segments), and each RNA segment forms its own RNP structure. The RNP strands usually exhibit loops at one end and a periodicity of alternating major and minor grooves, suggesting that the structure is formed by a strand that is folded back on itself and then coiled on itself to form a type of twin-stranded helix. The RNPs contain four proteins and RNA. The major protein is the scaffold protein, nucleocapsid (NP), and each subunit associates with approximately 20 bases of RNA. Associated with the RNPs is the RNA-dependent RNA polymerase (RDRP) that transcribes the genome RNA segments into messenger RNAs. The RDRP complex consists of 3 polymerase proteins, PB1, PB2, and PA, which are present at 30 to 60 copies per virion. The RDRP complex carries out a complex series of reactions including cap binding, endonucleolytic cleavage, RNA synthesis, and polyadenylation (reviewed in Lamb and Krug, 2001). Influenza virus infection is spread from cell to cell and from host to host in the form of infectious particles that are assembled and released from infected cells. A series of events must occur for production of an infectious influenza virus particle, including the organization and concentration of viral proteins at selected sites on the cell plasma membrane, recruitment of a full complement of eight RNP segments to the assembly sites, and the budding and release of particles by membrane fission. Here, we review progress that has been made toward understanding these steps in influenza virus particle formation. Throughout this review, we use the term influenza virus to mean influenza A virus unless specifically stated. This is because most research on virus assembly has been done studying influenza A virus.