The first part of the book deals with several aspects of different viruses : viral structure, function, replication and interplay between the virus and the host. Six viruses are used as examples, four RNA viruses (HTLV-I, HIV-1, MMTV and coxsackievirus B4) and two DNA viruses (EBV and KSHV). The second part of the book is devoted to the use of the knowledge on viruses to practical applications and also to the characterisation of HIV inhibitors. Reviewing the results of research on different viruses is important since, although viruses possess vast degrees of complexity, they also share similar features. In addition, viruses are more and more used as models to solve molecular biology problems.