Publisher Summary This chapter describes the cauliflower mosaic virus (CaMV). CaMV is the member of the caulimoviruses that are the only plant viruses known to contain double-stranded DNA. Cloned viral DNA can be introduced directly into plants by rubbing the DNA onto leaves with an abrasive, provided that the bacterial plasmid used to propagate CaMV in Escherichia coli has been excised. Viral particles accumulate in cytoplasmic inclusion bodies, spread throughout the plant, and can be found in most cells at high copy number. These properties of CaMV provide a useful way to introduce foreign DNA (inserted in the CaMV genome) directly into a whole plant. Special features in the secondary structure of the inserted DNA can also influence the stability of the chimaeric viral genome by interfering with the transcription or translation mechanism of the virus. The cytotoxicity of a gene product expressed from a CaMV vector can lead to the selection of rearranged viral genomes, where the expression of the foreign gene is abolished or modified.