Process Analytical Technology in Upstream Mammalian Cell Cultures Park, Cheol-Hwan Jeon, Yunjoo Uhlendorff, Selina Pein-Hackelbusch, Miriam Lee, Dong-Yup Roitberg, Bernard D. Biopharmaceutical quality in upstream mammalian cell culture has historically relied on Quality-by-Test (QbT) paradigm, where fixed processes are verified by end-point assays despite limited process observability and substantial biological variability. This chapter discusses how Process Analytical Technology (PAT), aligned with the Quality-by-Design (QbD) framework, enables measurement-driven process understanding and timely control by linking the trajectories of critical process parameters (CPPs) to critical quality attributes (CQAs). We review fit-for-purpose at-line, on-line, and in-line analytical technologies used in mammalian cell culture and summarize their roles across the process development and manufacturing lifecycle. Applications include multivariate process characterization, state-based monitoring and control, and trajectory-based process supervision. Finally, we outline future directions toward IIoT-enabled connectivity, soft sensors, and integrated multi-sensor PAT platforms, which are expected to support adaptive control strategies, digital twins, and ultimately autonomous biomanufacturing. Elsevier 2026 info:eu-repo/semantics/bookPart doc-type:bookPart text http://purl.org/coar/resource_type/c_3248 https://www.th-owl.de/elsa/record/13712 Park CH, Jeon Y, Uhlendorff S, Pein-Hackelbusch M, Lee DY. Process Analytical Technology in Upstream Mammalian Cell Cultures. In: Roitberg BD, ed. <i>Reference Module in Life Sciences</i>. Elsevier; 2026. doi:<a href="https://doi.org/10.1016/b978-0-443-24738-5.00086-0">10.1016/b978-0-443-24738-5.00086-0</a> eng info:eu-repo/semantics/altIdentifier/doi/10.1016/b978-0-443-24738-5.00086-0 info:eu-repo/semantics/closedAccess