Viral gene expression regulation refers to any of the processes by which cytoplasmic factors influence the differential control of gene action in viruses. The interplay of the viral genome with the host metabolic machinery involves modifications in both gene expression and regulation. Retroviruses have adapted themselves to use this machinery while maintaining the cell integrity, which is essential to preserve their survival. Consequently, there can be variable host pathogenicity associated with several diseases such as malignancies, immunodeficiencies, and neurological disorders. This book describes current research in the field, and gives a better understanding of the retroviral gene expression regulation that is essential to develop prevention and therapeutic strategies in the future.