The development and application of robust viral potency assays are critical for ensuring the quality, safety, and efficacy of viral vectors in cell and gene therapy. These assays play pivotal roles in assessing the biological activity of viral products across various stages of development and manufacturing, from early research to clinical use. This review explores key aspects of viral potency assay development, including the genome and physical, infectious, and functional titers, emphasizing their importance in meeting regulatory standards and maintaining product consistency. Additionally, a comprehensive overview of methodologies and protocols for assessing viral potency, with a focus on digital droplet PCR (ddPCR) and flow cytometry-based assays, is provided as examples. Flow-based assays evaluate the functional potency of viral vectors by quantifying transduction efficiency and transgene expression in target cells, highlighting their utility in assessing their biological activity. In addition, ddPCR provides unparalleled sensitivity and accuracy in quantifying viral genome titers, providing absolute measurements critical for quality control and regulatory compliance. Together, these approaches form a robust matrix for potency measurement, enabling the development of high-quality viral vectors that meet stringent regulatory standards. This work serves as a practical guide for researchers and manufacturers, supporting the consistent production and evaluation of viral vectors for clinical applications.