Because of the small size of their genome, viral genes have been forerunners in helping us understand gene expression. It is also because of their small size that viruses have elaborated the amazing variety of strategies that enables them to produce all the proteins they require for their multiplication. As a consequence, many of the strategies of expression known to occur in cell systems were first demonstrated in viruses. The aim of this review is to highlight the contribution of viruses to our knowledge of cell processes.