This chapter describes the range and diversity of virus genomes. Structural and chemical diversity, ranging from small, single-stranded RNA genomes of a few kilobases up to large and complex double-stranded DNA genomes bigger than those of simple cells, is unique to viruses and not mirrored in cellular organisms. RNA genomes may be affected by genetic mechanisms distinct from those which operate in cells and this is reflected in the replication and biology of viruses in ways which are discussed in subsequent chapters. By considering representative examples of different types of virus genome we can understand the range of genetic diversity seen in viruses.