Ubiquitin is highly conserved 76 amino acid protein found in all eukaryotic organisms and ubiquitin-proteasome pathway (UPP) plays a very important role in regulated non-lysosomal ATP dependent protein degradation. This pathway participates in or regulates numerous cellular processes, such as selective protein degradation, cell cycle progression, apoptosis, signal transduction, transcriptional regulation, receptor control by endocytosis, immune response and the processing of antigens. Nevertheless, roles of UPP in virus infection are only beginning to be clarified. Ubiquitin homology has also been found in insect viruses. All viral ubiquitin genes encode an N-terminal ubiquitin sequence and 3-256 amino acids C-terminal peptides. Most of the residues known to be essential for ubiquitin function have been conserved in the viral variant. In Autographa californica nucleopolyhedrovirus (AcMNPV), viral ubiquitin is attached to the inner surface of budded viron membrane by a covalently linked phospholipid and is not essential for viral replication. Currently, insect viruses are the only viruses known to encode ubiquitin. However, ubiquitin also plays a role in the life cycle of other viruses. Host ubiquitin molecules have been found in some plant viruses and other animal viruses. Additionally, Africa swine fever virus (ASFV) encodes a ubiquitin-conjugating enzyme (E2) and a putative causal link between human immunodeficiency virus type 1 (HIV-1) and ubiquitin was established by showing that depletion of the intracellular pool of free ubiquitin inhibits the virus budding. Further analyses indicated that many retroviruses proteins which are required for efficient pinching off the virus bud contain a late domain. The core element of the late domain is a proline-rich motif (PPXY) which mediates the late domain to be ubiquitinated by cellular proteins. Recently, it has been shown that many retroviruses have developed mechanisms to escape the cellular immune response, to facilitate virus replication and to promote virus assembly and budding via host UPP.