Viral replication refers to the creation of biological viruses in the target host cells during an infection. Viruses should first enter the cell before viral replication could take place. The virus continuously infects new hosts by producing numerous copies of its genome and packaging these copies. The replication of viruses varies significantly and is determined by the type of genes involved. Majority of DNA viruses accumulate in the nucleus whereas majority of RNA viruses grow only in the cytoplasm. Viruses can only replicate in living cells. The host cell must supply the synthetic machinery, energy and low-molecular-weight precursors, which are required for the synthesis of nucleic acids and viral proteins. The nucleic acid polymerases are behind the replication of genome of viruses and they are also involved in producing genetic variety, which is necessary for evading the host's defenses. Polymerases and other replication machinery components could be used as anti-viral targets. This book explores all the important aspects of viral replication in the present day scenario. It is appropriate for students seeking detailed information in this area of study as well as for experts.