Despite being small (approximately 22 nt) microRNAs (miRNAs) profoundly influence tissue-specific gene expression by interacting with complementary target sequences in cellular messenger RNAs, impairing their translation or marking them for early destruction. Recent work has shown that tissue-specific miRNAs offer a versatile target that can be exploited to control the tropisms of gene expression vectors and of replication-competent viruses. The principle of incorporating miRNA targets into vector genomes to control their tropisms was first demonstrated for nonreplicating lentiviral and adenoviral vectors, with subsequent extension of these studies to replication-competent (oncolytic) picornaviruses, rhabdoviruses, and adenoviruses. In contrast to previous targeting approaches, miRNA targeting looks set to be applicable across the entire spectrum of viruses and gene expression vectors. Here we provide a critique of the literature relevant to this new and rapidly developing field of endeavor. We also examine the possibility of engineering viruses for expression of tropism-regulating miRNAs.