Establishing reliable in vitro assays for antibody activity is fundamental to demonstrating product potency, comparability, and consistency throughout development and manufacturing. Such assays must accurately reflect the antibody’s mechanism of action while remaining robust, sensitive, and reproducible. This topic explores the challenges of assay design, including selection of relevant biological readouts, control of assay variability, and alignment with clinical relevance. It examines strategies for assay qualification, validation, and lifecycle management, particularly in the context of process changes and scale-up. Reliable in vitro potency assays are essential for regulatory approval and ongoing assurance of therapeutic performance.