The structure and functional peculiarities of a wide range of viral transcriptional transactivators have been considered. Analysis of literature data, concerning with the principles of functioning has made it possible to divide the viral transcriptional transactivators into three major groups: transcriptional transactivators of large DNA-containing viruses (Herpes simplex virus, Epstein-Barr virus, cytomegalovirus); transactivators of small DNA-containing viruses (papilloma viruses, papovaviruses, geminiviruses, parvoviruses) and viral coactivators. The latter group was identified in different DNA-containing viruses (Herpes simplex viruses, papilloma viruses, human T-leucosis virus, hepatitis B virus and adenoviruses). The conjecture about specific function of metal-binding motifs in activator domains of certain transcriptional activators is discussed. The functional features of human immunodeficiency virus TAT transactivator was considered separately by virtue of its RNA-binding activity.