Viral replication is the term used by virologists to describe the propagation of biological viruses during the infection process in the target host cells. When used in the strictest sense, the term refers specifically to the amplification of the viral genome in the host cell. From the perspective of the virus, the purpose of viral replication is to allow production and survival of its kind. By generating abundant copies of its genome and packaging these copies into viral capsids, a single virus can give rise to multiple progeny viruses.

The genome of viruses can be a RNA and/or a DNA, double stranded or single stranded, positive-sense or negative-sense, circular or linear, segmented or continuous. Depending on the intrinsic nature of the genome and a number of factors, different viruses employ different strategies to amplify their genome. David Baltimore, a Nobel Prize-winning biologist, devised a system called the Baltimore Classification System to classify different viruses based on their unique replication strategy. There are six different replication strategies based on this system (Baltimore Class I, II, III, IV, V, VI).