 Viral vectors are promising tools for gene therapy and vaccines, with viral vector-based vaccines enhancing immunogenicity without an adjuvant and inducing a robust cytotoxic T lymphocyte response to eliminate virus-infected cells. Many types of viruses have been developed as vaccine vectors, each with unique features and parental virus-related risks, and genetically altered vectors have been developed to improve efficacy and safety. Clinical trials have provided important information on factors such as toxicity, administration dose tolerated, and optimized vaccination strategy, and this review highlights major viral vectors that are the best candidates for clinical use. This article was authored by Takahiro Yura, Kenji Okuda, and Masaru Shimada.