This paper presents an introduction into basic principles of antiviral therapy. At the outset, the essential features of virus-host cell interactions have been described to facilitate a discussion of possibilities for antiviral therapy. Subsequently, the most important groups of viruses are evaluated according to the underlying mechanisms of replication (Baltimore classification). The rather limited examples of successful antiviral therapy have been documented. Elementary problems remain unresolved: How can viral replication steps be intercepted without afflicting host mechanisms? Finally, hypothetical approaches towards the elimination of persisting viral genomes from the cellular genome have been reviewed.