Viruses are submicroscopic parasitic and infectious agents that replicate inside the living cells of other organisms. Virology is the branch of science concerned with the in-depth study of all types of viruses. Plant viruses are viruses that infect plants. A virus particle, also known as a virion, is an extremely small infectious agent. It is essentially a nucleic acid (DNA or RNA) enclosed in a protein coat called a capsid. Viral genetic material can be double-stranded DNA, double-stranded RNA, single-stranded DNA or single-stranded RNA. A virus is a small parasite that cannot reproduce by itself. Once it infects a susceptible cell, however, a virus can direct the cell machinery to produce more viruses. Most viruses have either RNA or DNA as their genetic material. The nucleic acid may be single-or double-stranded. The entire infectious virus particle, called a virion, consists of nucleic acid and an outer shell of protein. The simplest viruses contain only enough RNA or DNA to encode four proteins. The most complex can encode 100 - 200 proteins. The book has been comprehensively designed, incorporating recent advances in this fast-moving field. It also provides accessible information on viruses in compact form for undergraduate students in biology are related life sciences.