Publisher Summary This chapter aims to summarize the currently prevailing concepts concerning chemical structure and biological function of the tobacco mosaic virus particle, quoting in many instances only the last stepping stones toward present position. Most of the emphasis is on the protein, concerning the structure and function of which have more definitive data than concerning the ribonucleic acid (RNA). Although viruses differ greatly in their shape, size, and composition, they seem to show several common basic principles of structure and function. The detailed study to which the tobacco mosaic virus is being subjected in many laboratories can therefore be regarded as fruitful ground work for the study of unrelated viruses. The chapter also discusses that nucleic acid inactivated by U.V. light or by various chemical substitution reactions is almost as effective in combining with the protein to yield stable virus-like particles, which are non-infectious. In contrast, traces of ribonuclease decrease at similar rates, the infectivity of RNA preparations and their ability to enter into rod formation. RNA and protein of distantly related strains yield few stable big particles, while non-viral RNA yields almost none. Thus the co-aggregation of the two components of TMV is definitely a function of their structural fit, as determined by their amino acid and nucleotide sequences.