Adenovirus vectors are the most highly efficient vehicles currently available for gene transfer to mammalian cells. Their ability to transduce both proliferating and non-dividing cells allows in vivo gene delivery, but the wide spectrum of cell types infected by adenovirus necessitates a requirement for targeting, particularly if the transduced gene is detrimental when expressed in inappropriate tissues. Over the past decade, numerous investigators have examined tissueor tumor-specific enhancer-promoters as a means to transcriptionally target genes delivered by adenovirus vectors. We review here recent developments in adenovirus vectors including improvements in the vector backbone to maintain promoter specificity. In addition, we discuss the regulatory elements directing cell-specific expression of genes encoding telomerase, prostate-specific antigen, probasin, osteocalcin, tyrosinase, alpha-fetoprotein, surfactant B, and mammaglobin. Recent results using these regulatory sequences to target Ad vectors to cancer cells are highlighted.