Potency assays measure the biological activity of biological products, providing data on drug dose, quality, stability, and clinical efficacy. Cell and gene therapies (CGTs) are complex, requiring specific potency assays tailored to each product. Unlike biosafety assays, these assays must be developed and validated individually. Successful technology transfer (tech transfer) is crucial in the manufacturing and analytical methods, with key milestones in transfer, process assessment, readiness, and execution. As a CDMO, expertise in GMP potency assays and tech transfer is vital for clinical and commercial success. The presentation will cover case studies and best practices for potency assays in ATMPs.