Process Analytical Technology in Upstream Mammalian Cell Cultures 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