The ultimate aim of biochemistry, wrote Sir Frederick Gowland Hopkins (1933), should be an adequate and acceptable description of molecular dynamics in living cells and tissues. My ambition, in this eleventh Leeuwenhoek Lecture, is to describe some aspects of the molecular dynamics of viral functions. A specific entity which is unable to multiply could not be a virus. The proper activity of a virus, the viral function par excellence, is consequently reproduction. Yet viral reproduction involves, not one, but a whole set of functions, for viruses, despite their reputation, are highly complex structures. In order that the lecture should not appear as the expression of an esoteric and impervious doctrinal corpus, we have to state first what we understand by the term virus, a necessary step in view of the fact that this is the fifth Leeuwenhoek Lecture dealing with viruses. It will also be necessary to explain the meaning of a very few unfamiliar terms. This done, a virus will be introduced into a cell and we shall try to understand how it develops. An infected cell may either die or survive. Sometimes a new balanced cell-virus system emerges, such as a lysogenic bacterium or a malignant cell. The complex interplay of cellular and viral functions will be analyzed. Then the current views concerning the mechanisms by which a cell regulates its functions will be summarized. This will lead finally to a discussion of the relation of viruses to regulating systems in general.